Ok, heres a strategy i've tested around 5 times and hasent failed me yet against the AI. It allows you to have an excellent defense while still being able to assimilate around 20% of the ships that comes accross your path. Heres what you need:

2+Borg starbases, about 10+Assimilators, and a few of those technology assimilators just so you can get some special weapons. Put these around your base, assign about 5 Assimilators to a starbase and a few tech assimilators with one too. Make sure they all are on low movement autonomy and high special weapons autonomy.

It's also nice to make a shipyard right next to a few starbases so when the assimilators get done with something, you can throw a little crew aboard and get the ship fixed fast. Just use the starbases to put crew on the derelicts since they can recrew. Change around the tech assimilators after they have 2 new special weapons so you can keep getting new ones, use the assimilators with 2 special weapons during your attacks, they can be valuable.

Have fun  

And watch it get destroyed by 2 Excelsiors...this is just a fun strategy, just to mess with AIs. I've played with it and captured every races con ships and made an unholy super fleet and systematically taken defense points like wormholes and moons throughout the map. Then when I get bored I murder them.