Depends on what you mean.
Obviously, playbys that are against site rules are off limits. We don't want people using actual IRL awful people as playbys.
But other than that?
Go nuts. I've used Power Rangers actors, comic characters, anime characters, cosplays.
As for AU stuff, that's entirely up to who's willing to write with you. Everyone has a different tolerance for that sort of thing, but it's technically not against any rules.