Ynnwn are the only true "mixed" race, being an offspring of Diaboli and Elves originally.
Kran reproduce by gemmation, which means Kran only have one biological parent, and can't have children with other races (or even other Kran, as I recall).
Klyros lay eggs. Only Klyros can make more Klyros.
All the other races can have children with each other, but the child will take completely after one parent or the other as far as race is concerned.
I've looked into this multiple times, and this is what the lore says, and I've found nothing in or out of game that says otherwise.
I have seen, at times, people changing the character description ever-so-slightly. For instance, making a Lemur with slightly pointed ears, perhaps from a Dermorian, Nolthrir, or Ynnwn parent.
Hope this helps.
