1) no restrictions, but don't make a 30 min film, please.
2) you can, but I am really pushing for originality with this comp. If you can make it original, go for it.
3) I'd prefer that you spend your time making 1 kick-ass robot. If you find you've done all you can and have it all animated and rendered in a nice scene etc etc and still have a lot of time, I might consider it.