I think you are approaching this problem incorrectly.
You should be creating weak creatures that scale in level, with better summons either having better base stats, more attractive skill options, or better scaling.
This keeps the spell relevant for hybrids, as a level 5 Zombie is pretty useless by level 8. You are punishing hybrids for no reason.
That being said, if you are dead-set on this, you can cast a spell at a location and listen for it in script. Then create a unit from a unit template and level it up to the desired level.
