How the AI behave really seems to depend on a lot of things. I test out planesets to see how the planes react to each other in combat. I chuck the planes that don't fight the way I think they should, or do some .ini editing if I think that might help.
I've found that sound mods and audio hardware configurations, of all things, can affect how planes behave in combat. Even weps behavior can be affected by the sound setup. I tested onboard audio v. my Audigy 2ZS recently and discovered that enemy aircraft would actually self-destruct, occasionally, in onboard mode -- in mid-air. Funny eh? One of the 3rd-party sound mods I was using would cause A2A missiles to detonate prematurely. Weird. But they'd knock the enemy planes down anyway.
For a start, you might want to download the 'AI mod' here in the CombatAce Downloads section and see if that helps with AI aggressiveness. If that doesn't work, you might have to do some experimenting.