I'm a vote against epic, it take an already longish game and makes it longer.
You could do what you suggest with the monsters, but say you choose oozes, those 8 oozes aren't the same there's a theme there but the monsters are different. Also some monster play off each other which means you might not want to draw from them if playing your variant because they would be substantially weaker.
Ahhh, I didn't realize in the set of monsters they were different from one another. That helps quite a bit.