A group of Creepers is called a Fizzle (might be Sizzle, I can't remember for sure). I don't know if that's official or not. A group of Endermen is called a Haunting. Those are the only ones I have heard of before.
I've come up with a few of my own and shared them with others:
-A Squish of Slimes (same as yours, coincidentally)
-A Cluster of Shulkers (but I do like Lurk better, I'm going to use that from now on 🙃)
-I just thought of this while typing this, but a Field of Shulkers sounds cool as well.
-A Tribe of Zombie Pigmen
-A Swarm of Blazes
I really like Pop of Magma Cubes and Cloud of Ghasts. Props for such creative names.