Thats a good point, Maybee someone should make a mod. Id do It my self but I donno How...
That's a cool idea actually, reduce the standing fleet and gauss cannons at their base a tad, but add a cap ship with some unique abilities to make taking their homeworld a little more challenging.
If you don't want to model new ships for them and just use TEC's, it's very easy to do this.
-The pirate base defense template is defined in the GalaxyScenarioDef file, so you can edit it to create however many (or few) defenses you like. This includes turrets and ships that don't leave orbit.
-The pirate base itself is a separate entity file, similar to other planets. So you can modify its hp, bonuses, etc.
-The pirate raids are defined in gameplay.constants. You can easily add capital ships for them to raid with.
-If you want new abilities for those capital ships, you'd have to make them of course and plug them into their ships. This would be the hardest part.