I honestly haven't delved much into Epic Level stuff, but in theory I'd put a spell like that beyond the realm of normal magic, and that sounds like a decent working definition of an Epic Spell.
I would still make the spell costly to cast, expensive and hard to acquire materials, the attendant risk of having such materials. Might make it really bad for you to botch the spell too, either in terms of the created offspring, or personally.
Just my thoughts. Multiple templates are a really potent thing to grant, in general you won't (and shouldn't in my opinion) encounter a creature with even /one/ template, let alone more than one. I think templates are meant to be solitary things in most cases.
Anyway, just my spin on things. I confess to tending towards lower power games for the most part, but I've been DMing since 1979, so I've seen most every iteration of D&D since -- well, pretty much all of them.
A multi-templated critter should be a Mythic Foe, not one of a horde.