Maybe you should try it out and see if makes the AI as aggressive as you want it.
I know that even with a three good planet head start, I can only hold them off. By the time I start taking systems, it's too frustrating and I spend so much time producing ships for defense, that my R+D falls behind. With such a limited amount of planets, you won't have to worry about out researching the AI, I'll bet you will have a challenge just keeping up.
It is a VERY similar experience to the island hopping campaign the US Marines did in WWII. It is very difficult to dig the enemy out of a system, then keep it. As soon as I would take a system, the AI would push me out or I would have to abandon it to get my battle fleet repaired (hanging around with a damaged fleet will get your *** kicked on a small board). Many a crippled ship was left floating to be later destroyed by the enemy. I even returned to the system after limping home and repairing to be greeted by a minefield. A very surprisingly, unplesant surprise.
Maybe you guys should stop playing on YOUR terms and see if you can play on it's terms. The AI seems to like the small Boards. The medium was okay, but it got a little tedious by the time I met up with them.
Large Boards in any 4X games, give us anal retentive humans too much time to plot.
I hear so much "back off and let me build up my empire", then challenge me AI. Then you are dissapointed when it can't. I like the small Boards and apparently so does the AI. You want an "aggressive" AI, I just gave you the formula. It isn't so much a plea for help as it is an argument for masochism.
Try it, I am sure you will like it.