Enemy Group Spawns

I'm making a world and I've made an area, now I'm putting in the spawns. The problem I'm seeing, is that it's just a percentage chance for something to spawn as an enemy. I think I have a better idea.

You set up groups of monsters to encounter, instead of individual monsters. Now imagine there's a battle. The chance of battle is 100%. You have, let's say 4 different groups of monsters in mind. Each group takes a chunk of that percentage. So in this instance, 25% each. Although you could adjust it so one is an 85% chance and the others are a 5% chance, or anything in between. That way we could keep certain monster pairs guaranteed to spawn together, or to make sure certain pairs don't spawn together.

Although it would probably be best to make it something that could be toggled back and forth. It may be the answer some are looking for, but it may be a hindrance to others.