I also agree that the Evil tag isn't necessarily warranted for this spell. "Science!" has no room for evil.
Here are my suggestions:
We have a series of spells starting at 4th level, up to 9th.
Spell: We make the mechanics for the first spell simple enough so that the subsequent spells (II through VI) need only say "as hybridize beast I, except for blahblahblah"
Fertility: Fertility can be based on how many hit dice the resulting creature has, as compared to the spellcaster's (for instance, if the resulting hybrid is half or less hit dice than the creator, then it can, with other members of its species, breed true). Make it so that more powerful casters can easily propogate hordes of smaller critters, but be unable to keep larger creatures for breeding purposes.
Size: Make the main difference between the various spell levels be the size of resulting hybrid, as offered above by Geocorona, but loosen it up a little to allow larger-than-Huge creatures at the highest levels. We shouldn't need to add permanency or polymorph any object to creatures if we don't have to.
Lifespan: Find a simpler, more hard-and-fast equation for hybrid lifespan, but keep it based on the wizard's hit dice and other factors, like Geocorona's original equation did. That was a good idea, but the equation is almost too complicated.
Hybrids Created: Make the number of hybrids possible based on a multiple of the caster level (for instance, an 8th-level caster could make 16 1-HD hybrids, 8 2-HD hybrids, 5 3-HD hybrids, 2 8-HD hybrids, etc.). Or not... one hit die per level could be enough. Absolutely require that the spellcaster have a "source pair" for each hybrid to be created.
- Devon