The Cultist seems to mind his own business in combat
At the end battle on level 3, The Breach, I had a group of 2 cultists, 2 chanders and 4 gnarlings attacking the Lord and the Inhibitor. As soon as I removed the Rally All Minions flag, the Cultists would promptly decide to abandon the fight, perhaps picking on the enemy doors around, but not sticking to their current foe, the Lord or his Inhinitor.
Pence is the best.
Rally flags apply this behaviour already because it engages an AI mode where the unit is looking to fight. If they're not actively looking to fight they'll consider their needs much more.