[@King Cosmos]
Right now, the level of power wouldn't be too different between a slime and a wisp. But more magically focused creatures would obviously gain passives like "Magic Damage Boost" or something over time as well as raw stat changes, so yes certain creatures would be more powerful with the same spells even if they also had the same MP pool.

As for varying your toolbox and such, that's perfectly viable too! Raw power, versatility, specialties...any and every option, within reason, will be available!