In short: don't be abusive. Dungeon Keeper is still limited on stuff which can also affects battles. If there's too much things on a level, creatures stop dealing damage in combat. If there's too much creatures, no more creatures are created. If there's too many pathfinding algorithms or if they're complex, the game may freeze in your face, especially with 3 or 4 keepers on a level or in a labyrinth-kind map.
Generally, you don't need huge rooms. Having a "shitload" of gold in your gigantic Treasure Room is often unnecessary, having a too big Workshop results as having tons and tons of crates which add to the things limit, not talking of the already machines in there. A bit the same thing goes for the Training Room and Scavenger Room. Hatcheries can be stretched alongside other rooms without exaggeration.
I like dragons! They're the center of my life! I'll never forget them...