Jason I like enemies to fit the level theme, if there is a level theme. For that you'd need various themes and then populate from a list coinciding with that theme. So you'd need lots of monsters, some of which only show up in certain themes, maybe some that only show up after a certain level.
You can cheat a bit and have the white mice show up in snow level, the brown ones in a forest, etc...merely a reskinning of the monster. As long as you have some that are unique to each theme I think you are okay, and maybe one per theme that only shows up after a certain level.
Like The Snowman only shows up in the snow levels, and the Yeti only shows up in later snow levels.
The Bat is a cave level only, the vampire bat comes in later caves.
Imp is in lava/hell/fire levels, and the demon in later lava/hell/fire levels.
The soldier in the castle, the knight in later castle levels...
So that's 4 themes, 2 uniques per theme, 1 lower level and 1 higher level...You can even string 3-4 levels of the same thing together and then face the boss of that 'theme' or 'zone' before moving to the next.
Just spit balling here. Good luck Jason.