Well, obviously you don't want to cast another artist's work without permission... And I am not even going to go near the rabbit hole that is the whole "Custom vs Bootleg" argument.
I think when it comes to sculpting your own content, the sky is the limit. I think where most guys have success, is finding an obscure character from a B movie, or a portmanteau of a theme and character to make some kind of comedic statement. If you want to do demons inspired by something else, and want to sculpt them yourself, I see no reason why anybody would have shit to say to you about it.