NESS (Neshke Narovken Extendable Spawn System) could allow you to set this kind of thing up pretty easily, IIRC.
Here is a link to one of the first hits that came back on a search but it isn't necessarily the latest version. From the testing I did with it, NESS didn't seem to be so much of a spawn system (in a conventional sense) so much as an area populator. It does the kind of thing you mentioned. The method has its drawbacks though it's probably more flexible than I recall.
If you were to use it, you might want to consider something that applies the
EffectCutsceneGhost onto the creatures that're spawned. If you have a lot of creatures moving around and/or fighting at once, EffectCutsceneGhost can
dramatically lessen the resources they take up. A little more information and a demo module in
this thread. The creatures will be able to walk through each other but when in combat they'll assume the appropriate attack distance and maintain it. The other way to cut down on resources all those guys would use is cutting down on sound resources they play, oddly enough. That one I don't have a turnkey solution for off the top of my head but an oversaturated scene sound-wise is a known bottleneck. There are a few other tips and tricks for pulling off crowd scenes/big battles you can find in various threads in the
Omnibus. The two biggest ones, IIRC, if you're just using default AI are pathing and sounds, tho.