Random Monsters Map Patcher – diverse neutral armies.

It seems like composite armies are already being generated...

Even if they are being generated, I haven't noticed (or are you confusing Heroes of Might and Magic with NCF?). But this mod doesn't generate creatures in the simple sense; it replaces already generated ones on existing maps. That is, if a mapmaker placed an archer, even that archer can be regenerated into an archer and a pikeman.