With how I'm letting people pretty much do anything they want (to a point) with their own personal stories for their volatikai, I finally decided this will be fine. In canon it's super freakin rare, but it doesn't have to be for you guys. I won't have set times, like I had planned, for when they can have mixed offspring. That just sounds silly to me. You'll still have to show me their designs and get the okay from me, though!
I still do not allow volatikaiXany-other-species for offspring. I at least want to keep that true to their world. Otherwise it will get too confusing.

Terms to know:
Female human with male volatikai: volatiman
Male human with female volatikai: humankai