I've only watched the movies and cartoons (though not all of the cartoons), so I doubt I know much more than you.

I wouldn't play too many characters, as it's all about character depth, and that's hard to deal with if you have too many characters, but if you want to do two or three, that's fine.

Most mutants will be OC, but if you want to play a mutant from the X-men universe and think you can do it justice, that's fine too.

Power level and what their powers are don't really matter, aside from their effects on the person, as it's not a fighting RP.  It doesn't really matter if you want to play a guy with no real powers but a few physical mutations or an Omega class guy that binds himself to the physical world and matter so that he can go to group therapy.  Either are fine as long as you can play it realistically.