Edit:Still sticking with my vote but we could keep one normal, like a hostage/successor/pet/spare. Then again we already have plenty of successor options, Cerulean being untrained but appears ideal once moulded, Gamington is close second, Chemista could take the reigns again but I dont think anyone would like her methods, Psysquid is hardly in fit condition to lead, plus many new ones that may appear.
Not like it will happen just bored and figured someone else may have views they don't mind sharing, if not just ignore this.
I like the idea of having a 'supply' of at-hand people/animals to mutate. Considering our standards.... would schools of fish be acceptable?
Think about a large fishtank in the lair, or several of them, each with 10s or 100s of fish (even small fry), and each set up to be instantly all mutated, the entire tank, with some sort of mutagen. Pick an air breather to give a chance of some of those fish being able to function out of water - have these tanks be close to the sea areas of our lair to give water-only instant mutants a chance to flop to where they can function.
Have that as an emergency system - and as an 'at hand' source for more specific mutations.
We don't have to mutate humans to get good results. We can mutate fish to beast men, or to beast-beasts, then mutate the survivors from there, getting up to the level of minion we desire, whatever that is. And we can do it in something of quantity, 10s at a time, 100s, whatever. If we're doing something with low chances of success - alright. There's still -plenty- of fish in the sea. We just need to go catch some alive.