My stategy is to avoid diplomacy. Eventually I will start to knock the stuffings out of the AIs and 1 of them will become my friend. Culitivating a friendship is dificult because you might get a mission you can't complete (detroys defenisve structues when there are none).
Focus instead on the basic game play. The AI gets a resource bonus to make up for stupidity. you have to compensate for that.
1. REX (Rapid Exspansion) in the early game while avoiding exposed far flung colonies.
2. Don't waste resources in the early game. Don't buy what you don't need now.
3. Capital ships are best for early game. Buff their hull and keep them repaired. Get a second one if possible.
4. Frigates are fragile and could easily be a waste of resources. The credits may be better spent on another capital ship, or hull and armor research. My first fighting frigates typically support the cap Ships. IE. Point Defense and the Hosiko.
5. Home worlds get a tax bonus. Consider raising the infrasturcture level from 3 to 4 or population cap research when max'ed out. When to do this is a balancing act. It takes a while to recover the investment, but it can be done earlier and cheaper than space ports.
6. The AI gets a resource bonus, SO MAKE THEM WASTE IT! Kill the targets of opportunity, especially CAP ships. Draw their cap ship in to battle with yours. (You did buff the hull, or armor, or shileds?) They will stay and fight if there is hope. Wait till its weak to pounce with reinforcements of light frigates and then pursue it to death.