[Nyx]

I totally ignore the fact that they are different AIs. I'd just find out which warp points they're coming at you from and guard those regardless of who is coming through. Park the best warships you can there and kill everything. When you've got a solid defense that they don't seem able to get through, send a fleet through the most active warp point and take the system on the other side, killing everything inside and recolonizing or invading all the worlds and locking out the warp points in that one. Repeat as often as needed.

Nearly all the AI players are helpless against seekers. You can hit and run against them in tactical combat and they just can't cope with it. Even if they have one level higher missile tech than you, if you're moving away and they're chasing you, you'll rip them apart without taking damage. 3 destroyers with 2 capital ship missiles each and some armor can hold a warp point against up to 5 pre-shield AI-controlled light cruisers usually without taking any damage at all. 5 of those destroyers should be able to wipe out an entire system before they need to refuel.

If you have boarding parties, this is a great opportunity to get some wonderful new technology and free warships. Two frigates with boarding parties and missiles should be able to guard a warp point against any unshielded or barely shielded enemy fleet of 4 or fewer light cruisers. After a few battles, you'll have a big attack force of your own to counter attack with.

I use warships over bases and fighters to defend because they can become an attack force the instant my enemy falters. Bases, satelites, mines, and fighters cannot counter attack. But they are often superior defenders, so you need to decide how aggressive you are feeling when judging which option to use.