Sports-videography IMHO is no different to any other kind of videography. To achieve smooth motion the rule of thumb is - just like you wrote - "shutterspeed = 2 x framerate". If you use higher shutterspeeds, especially with fast movement of the subject or the camera, the result will be strobing and what you descibed as flickering.
I did a lot of tennis-videos the last couple of years (not with my DSLR though) and almost always used a shutterspeed of 50 @ 25p (or 50i - to be more precise). The max. shutterspeed I found useable was 100. Higher than that gave me too much stuttering for my taste and needs.
Concerning DOF: I'd guess it depends a lot on what you're about to do with the footage and what you're aiming for. Shooting sports with a DSLR is very hard, *because* of the shallow DOF and the not very ... well ... easy handling ... If you want to achieve a "filmic" look, shallow DOF is a must, but very hard to maintain focus, especially with a 50mm on a crop-camera (on the T2i the 50mm is an 80mm on a full-frame, therefore a "telephoto" lens).
I'd try to use the highest f-stop possible, to get the narrowest DOF. Even with f8 or so you'll get more "filmic" pictures than on a camcorder with a much smaller sensor. A wide-angle lens would be perfect.
Hope this helps a bit...
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