Ignoring the debate about the proper and responsible thing to do (and that train wreck of a thread about guests and waiters ruining wedding photos), I'll take a stab at providing some technical advice.
- Yes, each time you start and stop recording it saves a new clip. Furthermore, the 7D has a 12 minute maximum recording time, after which it will stop recording and you need to tell the camera to start again. This is generally not a problem because most videos are recorded as shorter clips and then trimmed and reassembled in post processing.
- You will need some kind of video post processing program. There are free/cheap options such as Windows Live Movie Maker and Apple iMovie, and then price goes up (way up) from there. Theoretically, as a one-off, you could use the free 30-day evaluation of Adobe Premiere CC. But also, there is a learning curve involved in gaining sufficient familiarity with the software, and with video editing in general, that should not be discounted.
- There is also a learning curve involved in shooting video, some of the major points indicated following.
- It is important to perform manual white balance for each lighting environment before shooting, otherwise you will go nuts trying to white balance in post. You will need a grey card to measure the white balance and then follow the procedure for your camera.
- The Canon 7D does not track focus while recording. Focus is calculated once when you start recording and if the distance to the subject changes you need to adjust to keep them in focus. On the 7D I believe you can use the back button (if setup) or half-press the shutter release to refocus, but that often results in ugly focus hunting in the video. Most videographers rely on manual focus. Manual focus will require considerable practice to do smoothly and reliability, and most videographers use either a magnifying loupe/hood on their camera LCD or a larger external LCD so they are better able to track focus.
- Setting exposure for video is different than stills. First you choose your resolution and frame rate, typically 24 or 30 FPS (and do not change these settings throughout the event). Then you select your shutter speed, which is typically 1/(2*FPS), so for 30FPS you would generally choose a shutter speed of 1/60. You can go with a faster shutter speed but this can lead to jerky looking motion and requires more light. Next you choose your aperture based on how much depth-of-field you want, and finally set your ISO to get properly exposed frames at the selected shutter speed and aperture (using neutral density filters or adding light if needed). These settings must all be configured before you start shooting, and only the aperture can be varied while recording. You will have to pay attention to exposure and color balance as lighting conditions change.
- You might need auxiliary lighting, such as a 200-300 LED light panel. Because you'll be using this inside it should be tungsten (3200K) or bi-color to balance better with indoor lighting, and should have a CRI rating of at least 85 (89+ recommended). And either a second person to hold the light or some kind of bracket to adjust it over the camera so it's not dead-on to the subject, causing flat lighting and red eye.
- Then there's audio. The camera's built-in mic will be worthless in this environment (pretty much in any environment) unless all you want is the noise of the crowd and the sound of your breathing and making camera adjustments. A decent shotgun mic such as the Rode Videomic will be better, and can be mounted on the camera (though you should use some kind of bracket to move it up and away from the lens to minimize noise) and plugged-in to the camera mic input. Best would be to use the shotgun mic on-camera plus putting wireless lav mics on the bride and groom that go to a separate digital audio recorder, with a separate recorder plugged in to the DJ console. You will have to monitor and adjust the audio recording levels throughout the event as well.
- You will need stabilization for the camera. The camera/lens IS will only take you so far. A tripod is one solution but is clumsy to drag around; but if you do use a tripod you need one that will allow you to pan and tilt the camera smoothly. A ball head is not ideal for video. A quality pan/tilt head would be better, and a quality fluid head the best. The tripod will need some decent height as well or your video will appear to be from a child's perspective. Most videographers use some kind of shoulder or chest stabilization rig and practice smooth movement.
- Don't forget extra batteries and fast SD cards for cameras, recorders, mics and lights.
When shooting get close to your subject. Avoid excessive panning and zooming during the clip. Shoot stuff that is interesting and shows action; the still photographer will take care of the static shots and formals (although some clips of the formals as they get setup can be interesting). You can always add stills to the video when editing. Most of the weddings I've attended has the videographer go around and ask guests to record a personal message to the B&G. Concentrate on highlights such as the processional, ring exchange, kiss, receiving line, introduction, toast, cake cutting/feeding, garter, bouquet, dancing. For some of this stuff you may well find your 100-400 a benefit, although the narrow aperture will cause some challenges with available light. Of course you're going to have a conflict on how to film yourself walking your daughter up the aisle and giving her away, daddy's dance, and the potential emotional moments of you watching the ceremony and sharing the experience with your wife and others.
I suggest talking to the hired photographer first and let him know what you've been asked to do, and ask for any advice or ground rules he can offer to minimize and interference between your and his needs. For example, your video light or even presence can interfere with his shots, and you will often be contending for position for highlight activities.
Filming the day is less than half the work. I spend at least 10 minutes editing each finished minute of video. My daughter does video post for a living and is much faster, but it still takes her at least an hour to edit a 15 minute video (and that's with the someone else pre-selecting which clips will be used). You might consider farming out the post production; there are many people from countries in Asia and Southern Asia who offer editing services and affordable rates. A big problem is deciding what to include, what to leave out and how to cut the clips to wind up with a nice result that isn't too long. Breaking the day into several different finished videos helps, but that also means creating chapters and a menu on the DVD, more work.
Based on the questions you've asked you need to spend a lot of time practicing and experimenting before the wedding, and probably a few hundred more dollars of gear to buy to do it well. Maybe it's time to take an honest, objective look at what you want to accomplish and why. You've already spent around $1k for a new lens, then you'll spend at least another $150 for a shotgun mic, possibly more for a suitable tripod or stabilizing rig, plus whatever software you buy. You might find that the same amount of money could have better been spent in engaging the photographer to also have a videographer cover the wedding. Then you could simply bring a good point-and-shoot to grab some stills and video as the opportunity presents itself, and otherwise attend to your role as father of the bride and enjoy the day.