1) pirates' own capital ships can get technology that they stole, bought or otherwise from another race.  Their cap ships might have to have capped abilities, so less multiple abilities than other races' ships.

2) pirates stealing a non-protected cap ship is cool.  It SHOULD be difficult to do and only possible if the race is dumb enough to leave one unguarded; BUT, the cap ship should not be fully functional, which can be remedied by offering something to the race the cap ships was stolen from or otherwise.  For example, the pirates steal a cap ship level 3 from the advent.  They can try to ransom the ship back OR ransom techs and upgrades with the pirates who CANNOT build one of their own but can utilize the stolen one.  So it may be that the pirates say "look, give me the ability to unlock this cap ship ability and 1) we will not use it to attack you and/or 2) we will ignore orders to attack you for a specified amount of time.

3) pirates should have some level of self preservation so that they know when to leave.

4) working with the pirates in whatever way softens their hatred of you meaning perhaps they leave your grav well earlier than usual or go for less important targets (you know, keeping up appearances), but that could come with diplomacy penalties when other races (even your allies) realize this.