Newbie co-op vs. AI... Help!

[Syllogism]

We (I'm the other player) aren't looking for a magical trick to destory the AI instantly, just a few hints so that we actually can have a game that is a challange ( note that, to me, a challanging game is one in which victory and loss are uncertain for much of he game.).
As it stands, we're geting ripped apart by medium ( not too fun to lose by turn 40) or are shreading low bonus AI without breakign a sweat.

a slightly more detiled description of our basic stratigy woudl be:

1. expand heavily (both towards for unified boarders and away to ensure as much territory as possible)
2. continue to expand and build up insustry/research until we contact the ai
3. defend chokepoints
4. once we'rre strong enough, start whittlig down the ai with our (hopefulloy) superior fleets.

Basic fare for a game of ths type, really.

Defensively we're using misile/pd ships as the core of the fleet (especially me since my beams are nearly useless pre talisman) and using them to hold chokepoints. The problem is that they build larger, more technologically advanced flets ( that while slightly less well optimized are generally more then up to the task of obliteratign us)

Should we be working with a different stratigy? like rushing t fighters for defense or mines?
should we just rush he ai ( somehow)?