To spawn enemies, try with the arena world that @Froggy provided:
Not sure that it still works, since I remember having issues were it would only spawn one team, but…
You can use console in the worst case to spawn the other team. Or maybe I had this issue on the upgraded version which reads the data from a json file, not sure where did I get it from 
In short, you have to retrieve the population of your enemy faction, and use create citizen with it.
If this microworld doesn’t work for you, try doing that.