There's also a huge damping variable due to communication latency between star systems, and of course travel time if you want two armadas to gang up on a third system. In fact this is exactly the two generals problem: you really don't want to launch an armada without confirmation from your ally, and if you do launch it, you will have the two generals problem recalling it.
Yeah, it would be interesting I think to add an aggression variable. If two species meet and their combined aggression is high enough, they fight (with some large bonus going to species with some combination of aggression and ability to detect, as it is assumed they can get off a devastating first strike).
If the species are not aggressive, they can presumably at least the weaker of the two should get some large bonus.
Of course then we could play with the first strike bonus and the alliance bonus, and get whatever result we want from the simulation.
https://en.wikipedia.org/wiki/Two_Generals%27_Problem