I suspect that the problem you are experiencing is Magic Toxicity. This can be monitored in your slaves by using the Mind Read ability. I don't know of any mods that alter the behavior of toxicity or reduce the amounts found in items.gd.
The primary way to avoid random mutations from toxicity is wait many days between applying potions to your slaves. Each majorus potion gives 30 toxicity and each day the slaves randomly lose between 1 to 5 toxicity. When a slave has more than 60 toxicity, they will have a 25% chance of mutating. As the toxicity increases even further the number of mutations increases(1 per 30 toxicity). The core of the toxicity mechanic can be found in Mansion.gd and the mutate effect is found in spells.gd.