at least one AI always builds a massively larger military than any other civ. maintenance for them never seems to be an issue.
It recalls me the "stack of Doom" in Sid Meier's Civilizations 3/4 ...
AI always goes aggro randomly and for absolutely no (apparent) reason.
game always snowballs into an arms race because of these.
It sounds like in real life, don't you think ?
But yes, the balance can be difficult to find in a game as you decide who will be against you. It is like a cocktail, if you end up adding civs with high-testosterone, they will wage war asap, and research accordingly ...
And maintenance is not an issue: trade and wealth building are enough, especially if you know how to use the adjacency bonuses when placing your buildings. Don't forget that you can make a lot of money sending freighters to another civilization (always try to reach the max for your trading routes) , and trading techs (with the right civ!) is also very lucrative.
It is also why the diplomacy is often underestimated by players. I play at godlike level and the only way to survive the 1st 100 turns is to have diplomacy with trades (freighters), ideology, government (+2 diplo with Interstellar Republic). And you will see that the civs will like you (a bit) more.
But yes. There are really some civs you don't want to invite in your game if you would like more peaceful game. Avoid malevolent civs if you can, they are the most difficult to play with as players often tend to be pragmatic/benevolent, and do not try to boost the diplomacy bonuses.