Hmm, the floating creature (fire skull and wraith) would work no problem, but the walking ones would need their stances slightly altered to appear to stand properly on a platform. I drew these with a isometric perspective in mind, so you'll notice in any Left and Right facing humanoid sprite that the leg furthest from view appears slightly higher to suggest that kind of perspective. I think just dropping that leg down to match the closer leg, and then erasing any ground shadows, would be all it takes for the humanoid sprites to work properly for a Platformer game.