i agree with that too, because roleplaying out the same old myths is kind of pointless, and boring, really.
muses are revered as demi-goddesses (i believe), so i don't think that would be a problem.
drawing a line at whats in and whats out would be rather hard; but I think finding and trying to translate personalities and really getting to the heart of a character when they're a seven-headed hydra might prove challenging; so I think we'd have to draw the line at maybe gods, goddesses, demi gods, and maybe even heroes? ive never really seen someone pick up atalanta, or achilles, or hercules, or bellephron -- but i think there would be enough individual substance to do so? the muses are greek goddesses, so they'd be fine!