Random Monsters Map Patcher – diverse neutral armies.

It's still better not to mix creatures from different factions.

If they are all from the same faction, it won't add much variety. But at least, I think it's worth separating them into "good" and "evil."
For example, if the AI can use combinations like: Inquisitor - Light Unicorn, that would be great. You can increase the weight for triggering these combinations. ;)

Added 17 minutes later
Combinations:
Knight + Archer,
Cyclops + Goblin,
Shaman + Goblin,
Obsidian Golem + Archmage,
Yellow Gargoyle + Archmage,
Red Lich + Vampire,
Mummy + Souls,
Light Unicorn + Adept,
Nymphs + Ents,
Obsidian Golem + Priest 1st level
etc...