In the encounter, are the rats your enemies? Because if they are not, then cleave is not even relevant. You attack one enemy, if you hit you can do paltry damage to another enemy.
What is being discussed borders on the absurd. Do we need rules that say that your character needs to breath to survive? And if we do, does it mean that the rules should also say that they need breathable air, not water to survive? No because it is unnecessary.
This is only a problem with DMs that allow absurdity in their games and play with absurd players. I'm pretty sure there is no need to add a bunch of rules for every single stupid idea that a player can come up with. If we do need those kind of rules, it's no wonder that the RPG market is shrinking.