There's no evidence that Mystics are descendants of the Reptites. Kilwalas were already present on the Mystic Mountains in the Prehistory. And the creatures in Giant's Claw are all reptilian.
I really think it's too illogical for the Demi-Humans to be related to Dragonians. Humans and Dragonians are nothing like lions, ants, dwarves, mermaids, fairies, etc. We don't even know if a cross between the 2 species is biologically and socially possible... they did try to slaughter each other in a deadly war, and the goddess of the humans humiliated the god of the dragonians.
And tails don't count. Dwarves and fairies don't have tail... and cats do.
Not only are the Demi-Humans too different from humans and Dragonians, but they are also too diverse. I think it's clear that Demi-Humans are not a single species but just a collective name for people who aren't humans, Dragonians, or unintelligent monsters.