Additional scripts, plus a life simulating engine that interfaces with the rendition engine (but I don't know anything about Unity). Something more elaborated than just idle - patrol - scout - attack.

I have no clues about the additional size in files and required time in development, not knowing anything about Unity, but I don't see how it wouldn't be doable. Looking at other games can only lead to max requirements (for example, Stellaris has about 10 MB of scripts to animate a small galaxy (20 MB, including events management), in about 1000 files: that's several 100,000 lines),

To simulate intelligence, I believe that neural networks are not necessay. Fuzzy logic or ponderated random decisional trees can be used to introduce some variety and chaos in otherwise scripted detailed actions.