Here's a racial lineup.
Streamies = The majority race in Waylights, if you don't count Tsuxia. Streamies are mostly human, with a few mixed Invader genes into them. The base race (human equivalent).
Plats = Primarily Plateau inhabitants, although there are quite a few around the Slipstream. Plats are unmodified or largely unmodified pre-Invasion humans; they are shorter and slower than Streamies but have greater strength and endurance. Dorf equivalent. Tsuxians consider plats to be the only true humans, to them streamies are mutants and everything else is an Invader.
Geeps = Descendents of intentional hybrids between the alien overlords and humans. While they are weaker than both plats and streamies, they tend to display greater finesse and psi attunement. Basically an elf equivalent.
Womps = Descendents of hybrids between the alien footsoldiers and humans. While stereotyped as stupid, they generally simply have a more direct approach to things. While they do not have a plat's endurance, they are extremely strong. Basically your half-ork or ork equivalent.
Interbreeding is possible and indeed has been going on for fifty generations; at this point genomes have stabilized and children will breed true to one or the other parent.
Two more races exist that are considered semi-sentient, gorks and lemlins. Gorks are feral Invader footsoldiers that have managed to return to sexual reproduction; they live in a pre-civilized state in the deeps (Ogre equivalent). Lemlins descend from Invader technicians who have also managed to return to sexual reproduction -- the shallowness of their gene pool has taken a toll however, and while they are clever and move extremely quickly, they're generally nonverbal and most lands do not consider them people (Goblin/kobold equivalent)