It's time to show off your creativity a bit more in this forum. Tell us how you handle race backgrounds in your homebrew or changed things a bit in Golarion to fit your vision of the planet?
What I did:
Probably the best thing about integrating the Psi Races from PSIONICS UNLEASHED and the XPH is that they aren't integrated in Golarion in any way. So I turned to my campaign's standard of explaining how they are wandering Avistan.
-- Elans are part of a secret society since the psionic process of their creation is similar to a psionic lich's. In both If Thoughts Could Kill and Hyperconscious, a Psionic Lich is the result of altering a body with psionics instead of magic -- the necrotic flesh is powered by psychic energy instead of negative energy (although I'm sure the universe doesn't know the difference). For an Elan, your transformed in much the same way, except you are still living.
-- Maenads are the direct descendants of the survivors of the Explosion in Azlant. They have crystalline formations in their skin as a reminder of that explosion. They have different physiologies that allow for crystallization of various substances on their skin -- say sugar or certain human pheromones and hormones.
As for Elves, Dwarves, Humans, Gnomes, Half-orcs, Half-elves, and Halflings; I stuck to the regular explanations in my Golarion campaign. On my world, though, Halflings and Gnomes inhabited the islands, the Dwarves reside in mountains, and the Elves in forests.
Although I admit that I'm exploring racism between elves because of a historical thing happened many years in the past that caused the elves to split.