I agree about AI Focus... forget it even exists. Maybe it's better on more recent cameras, but it's basically not a focus mode at all. In AI Focus the camera is supposed to decide for you whether or not the subject is moving, then switch to using whichever is appropriate: One Shot or AI Servo. I haven't touched it in years, but when I did try it I found a delay and too many mistakes, missed focus on too many shots. Maybe it's better on more recent cameras, but I wouldn't know. I like to make my own decisions and not leave it up to the camera to decide.
Your 30D has 9 points, with only the center one the more sensitive and faster/more accurate dual-axis or "cross-type". So, doing what you are doing and using just that point is probably wise, particularly for moving targets.
Your camera doesn't have Expansion Points, so forget about that. The only other choice you have is "All Points", which again leaves it up to the camera to decide where it's going to focus... and that can lead to a lot of errors.
Basically, you need to be in AI Servo any time the subject is moving. And you need to work hard to keep the selected AF point on the subject, right where you want the camera to focus. This leaves you in control.
One Shot is fine with stationary subjects. It focuses when you half-press the shutter release button, then when focus is achieved it stops (or locks), and gives you Focus Confirmation (the active AF point flashes red, the green LED in the viewfinder lights up, and if it's enabled the camera will emit an audible "beep"). Focus remains where it was achieved as long as you maintain half-press on the shutter release button. If the subject moves or you move or for any other reason you want to re-focus, you have to lift pressure off the button momentarily, then reapply half-press to cause the camera to focus again.
In One Shot, you can easily use a focus and re-compose technique, if you want to only use the center AF point and don't want your subject to be centered in every shot. As soon as you get focus confirmation, feel free to change composition if you wish. Focus will remain where you set it, as long as you maintain half-press on the shutter release.
You also can use hyperfocal focusing techniques with stationary subjects and One Shot. If you've set a small enough aperture, you can focus on a point approx. 1/3 into the scene and potentially have everything near to far in focus. (If you were to focus on too close a subject, the distant objects will be out of focus... if you instead focus on the distant object, the near ones will be OOF, unless you use extremely small apertures.)
With One Shot, the peripheral AF points are generally usable, since stationary subjects don't demand as fast focusing as moving ones do. So, another way to avoid a centered subject is simply to select another of the focus points, if you wish. This is instead of a focus and recompose technique, though you can still do some slight recomposing once focus is achieved, if you wish.
Using the shutter release button with AI Servo is different and doesn't allow for focus and recompose or hyperfocal techniques (read on, though... more about this below). Basically what happens with AI Servo, which is designed to track moving subjects, is that focus is constantly updated and never locks. You never get focus confirmation because focus keeps adjusting for the movement of the subject, as long as you maintain half-press on the shutter release. Yes, you will see the AF point flash red briefly, when in AI Servo. But it's not the same as One Shot. In AI Servo the point flashses when AF starts and to indicate which of the points is active... it's not Focus Confirmation. Notice that the green LED and "beep" never occur in AI Servo.
Since focus never locks in AI Servo, if you try to recompose or in any way reposition the AF point other than where you want the camera to focus, you will cause the camera to refocus onto the new subject or to whatever distance happens to be covered by the AF point at that instant. So the basic way to use it is to keep the AF point right on the subject you want in focus. That can take some concentration. It just doesn't work as well, to activate all the AF points and hope the camera tracks the subject from one to the next properly. So it's still best to use only one AF point, and the center one is the most sensitive and responsive, which is why many action shooters choose it to use (some later cameras have improved peripheral AF points that are a little faster and more accurate... 40D and later, 7D have all points cross-type, though the center one is still further enhanced "diagonal/cross-type". 5D and 5DII are more like 30D, only their center point is the "better" type).
So, since you can't do focus and recompose and you want to try to keep using the center point only, how do you avoid centered subjects when shooting AI Servo? Well, one way is to frame your subject slightly loosely and do a little cropping later, in post production. It's a bit of a fine line, with an 8MP camera.... You want to avoid cropping any more than necessary. The later 15MP and 18MP cameras give you a lot more flexibility to crop.
As Chris mentioned, many action shooters who use Canon really like Back Button Focusing or BBF
. This separates the AF function from the shutter release and reassigns it to a button under your thumb on the back of the camera.
By doing this, you have more control over AF. It's no longer part of the shutter release process. AI Servo can be used as a default focus mode, since it will work with either stationary or moving subjects. You can start AF before anything else, while tracking the subject and maintaining the AF point on it, then take your shot when you see the subject doing something interesting or posed as you want them. Plus, with BBF you can focus and recompose in AI Servo, or use hyperfocal technique, simply by lifting thumb pressure off the button and stopping continuous focus, so long as the subject isn't moving.
Sounds ideal, doesn't it? So after changing to BBF, why not use AI Servo all the time? Well, One Shot is still the most accurate method of focus (Live View is more-so, but not a feature on your camera). So there are still times I switch to it. Most of the time, though, I just leave my camera in AI Servo by default, using it for both moving and stationary subjects, along with BBF.
It takes a little practice, but not a lot until it's second nature. You might want to give it a try. Once I learned to use BBF, I never looked back. All my Canon are set up to do it and have been for years. It works fine for One Shot, too.
On 30D, it's Custom Function 4, "Shutter Button/AE Lock", set to option 3 (page 166 in your manual). This reassigns focus to the * button on the back of the camera (and on the vertical controls, if you use a BG-E2 on your camera). The * button normally provides AE Lock momentarily, but Option 3 leaves AE Lock off, which is a good idea in most cases. If you ever need AE Lock for some reason, you can either switch this custom function to Option 1. Or... even easier... simply temporarily switch to M/manual mode and set both aperture and shutter speed where you want them yourself (AE Lock only works in an auto exposure mode: Av, Tv or P... But M mode is sort of the "ultimate AE Lock" anyway).
Read the article about BBF linked above and experiment with it to see how BBF works for you. A lot of folks who try it and get comfortable with it never go back to the old ways.
No, on 30D, there is no change to the metering when you set BBF. It continues to start when the shutter release is half-pressed. In fact, I think metering also activates (along with IS) when either the shutter button or the * button is pressed. The only choice is whether or not it's using AE Lock for every shot (and AE Lock would not be a good thing most of the time).
One other "trick", stop your lens down slightly, to give yourself more depth of field "elbow room" for slight focus errors when shooting moving subjects. Using a large aperture lens wide open is more challenging to nail every shot, you will find more focus errors. Simply setting a bit smaller aperture often covers those completely.
After all the above, you might wonder if there is ever a time and place to use All Points/Auto AF Point Selection. Yes, there are.... One time, for example, is when shooting moving subjects agains a very plain or very distant background. Think of a bird or flock of birds against the sky, particularly if it's cloudless or, if cloudy, it's solid overcase or there is very little cloud detail to "distract" AF. Combined with stopping down slightly, All Points AF might work great in a situation like that.
Later cameras with more processing power can be a little better maintaining tracking and faster locking onto subjects. I got pretty good results with my 30D (Digic II).... saw a bit of an improvement going to 50D (two generations newer, Digic 4)... and another incremental improvement when I upgraded to 7D a couple years ago (dual Digic 4 plus a discrete AF processor). However, it wasn't improvements in leaps and bounds. The vast majority of my shooting is action/sports, using the techniques mentioned. I probably take about 50,000 sports/action images a year. With practice, I was getting about 90-92% acceptible focus with 30D, maybe 93-94% with 50D and now around 95-97% with 7D. To some extend, it's probably the additional practice... but I also credit the cameras. For comparison, I see much lower results - under 50% acceptible focus with the same moving subjects, lenses and situations - with 5D Mark II. I might do a bit better with more practice... but I use the FF camera mostly for stationary subjects. And have use the croppers 30D, 50D and 7D for the action shooting.
You'll next be at the mercy of your lenses.... which also effect focus speed and tracking ability. Those with a large max aperture, that provide more light to the AF system in the camera, are faster to focus and better tracking as a rule. Also, lenses with USM (or HSM in Sigma, USD in Tamron) tend to be faster, more accurate and quieter. Your 50/1.8, has a big aperture, but is known for slow, somewhat erratic and noisy AF. Because it has USM you will probably have best luck with your 15-85mm, even though it's not a large aperture lens. The 55-250 is somewhere in the middle, not USM so not the fastest, but often adequate for the purposes. Any of the Canon 70-200s would be faster, though. Your EF-S 60mm macro has a reasonably large aperture and USM, but most macro lenses are not built for speed and have to move their focusing elements a long, long way to go from infinity to 1:1. I don't have that lens, but some macro have a focus limiter switch, that restricts the range of focus to speed things up a bit. I dunno how fast focusing your kit 18-55 or the Sigma 30mm are, haven't used either of them.
