Hey Jake,
I think there are no real rules to looking for old lense.. so long as you can make it work without having to break your camera go for it...
When I hit the pawn shop \ thrift store I first check for modern EOS compatible lenses. Take some time to study the mouting end of your kit lens. In pawn shops and thirft stores YOU have to know what your looking for, half the time the sellers have no idea.
When it comes to OLD lenses, youll need an adapter for sure, which adapter depends on which lens mount you end up with. Some lenses will require an adapter that has its own lens in it. You want to stay away from that. For example, dont ask why, but OLD canon FD lenses will NOT WORK on the NEW canon cameras. You CAN get an adapter that will sorta make it work, but it has its own lens and doesnt work worth a darn.
I belive the most popular lens mounts for the T2i are Nikon and Screw Mount M42. Check the wiki for these lens mounts so you can regocnize them when you see em, again, your likely the one who needs to know when digging in the box of old lenses..
the adapters are Nikon to EOS or M42 to EOS and can be found in the usueal places.