Change:

When a Wizard Mutates a creature, they create a new creature with exactly one more or one less quality than the starting creature.

To:

As an atomic action a Wizard who can complete all of the steps may breed a creature.
#That wizard (known as the breeder) selects a creature (known as the stock) in a stable belonging to another wizard (known as the proprietor)
#creates a new creature (known as the whelp) exactly one more or one less quality than the stock
#spends food equal to the stock’s upkeep
#Adds the whelp in their stable, optionally removing a creature from their stable as normal.

remove:

A Wizard may mutate a Creature in another Wizard’s Stable by spending an amount of Food equal to that Creature’s Upkeep; the resulting Creature is added to the mutator’s own Stable.