I always thought of "Mystics" as a catch-all term for sentient non-humans (of obviously varying intelligence). Clearly there are many races represented in Magus' army, and there's no reason to think that humans and dragonians are the only species that achieved sentience through evolution. Given that the monsters seem to use magic, and Flea, and Magus, I doubt magic was the mystery to them that it was to the humans of that era, hence the name. I'm pretty sure demi-humans could be included in the 'Mystic' umbrella - wouldn't surprise me one bit, and it adds, as I've said before, an interesting and rather sardonic twist to Magus' curse on Frog. (Imagine that - he not only changes Glenn into a non-human, he changes him into his -enemy-. Admittedly, however, I just like the poetic irony in that.)
It would make sense, given the context of the game, for the demi-humans to be more closely allied with the draconians, given that the draconians were all about the Planet and nature and stuff whereas humans were (according to CC...) Lavos-influenced.