Sounds like you are well-equipped to shoot some portraits! If I were you and shooting with the 7D I would take advantage of the fact that you have ETTL available via the built-in flash as a master and use that to trigger the 550EX in slave mode, letting the camera make the metering decisions. Of course, if you aren't getting the result you want that way, you can always switch everything to Manual and dial in exactly the power you want on the flash (and set your aperture and shutter appropriately). Your results will be more consistent with everything on manual, but it's more work.
Either of your lenses should work well for portraits. The 70-200 L is an awesome portrait lens. I don't have the Tamron you mention, but I would work at the long end of that zoom range (at or near 50mm) because you get a more flattering look when zoomed in. A wider angle tends to overemphasize the nose on the subject, so you want to avoid shooting wide and close.
In the videos of mine that you saw I used the 70-200 f/4 L IS, and at other times the EF-S 17-55 f/2.8 IS, and with the latter I was almost always at 50-55mm for portraits (which on these crop-sensor cameras is about equivalent to 80mm or 88mm on a full-sensor camera).
Hope that helps!