From what I remember, ball doesn't believe in single player
Anyway, for the campaign I'd reccomend SLX over ACE. ACE is designed for multiplayer and a lot of it won't work without another player. SLX is another realism adding mod that works, in my opinion, better in single player than ACE does. Still, major mods that so drastically change the AI's behaviour may break some missions. You will have to be a lot cleverer about how you tackle the enemy because they'll chew you up and spit you out if given the chance.
I remember with SLX for ArmAI I tried the water tower sniper side mission in the campaign. Never had trouble with it in vanilla. With SLX my first few shots with the Mk12 downed 3 enemy resulting in smoke and suppressing fire being directed at me while two teams bounded towards me and another tried to drag the downed men to cover. From an easy 5 minute mission, it turned into a 15 minute desperate bout for survival that took me 5 goes to complete.
I havn't tried the ported version of SLX on ArmAII yet, which apparently has a lot of fixes. I'll have a go this weekend and let you know how it goes.