There shouldn't be restrictions, but there should be logic. If someone wants to be a part vahla part epixhinex part human part kiffar, why not, they're all close enough. They shoudl jsut keep in mind that if they do inherit any of hteir ancestors traits, such as the epixthenex's mental resistance, it would be decreased. Prehaps it woud only be half as strong if you're half epi (just going to call them Epi because I have no idea how to spell it), or only a quarter as strong if your one fourth epi, for instance. Why not, as long as it's not being abused.
But things like half genosonians half transdoshan... I hope nobody has ever tried anything like that. Bug+lizard=nothing. If two species are close enough to breed, then why not let people make characters of that combination, as long as they're not exploiting it to get more strengths or storngers strengths. As long as those are kept balanced why limit it more?
Tl;dr: Don't restrict it, just make people realize that they can't be one twenty-fifth every fancy near-human species and have every single one of their traits. But also don't let people be half Herglic half Yodaling.
Edit: There was like 5 new posts between me starting to write this and me clicking post... So a lot of the things I just said were said just before I said them.