A player who plays as a Pirate could start with an armed survey ship (with excellent range), some money and good diplomatic skills. Perhaps with an intro of which civlization conquered his minor home world.
Since the pirate upon starting the game has no home world, he has to trade with civilizations he comes into contact with to obtain technology / ships. He can then upgrade ships to get weapons.
When the pirate encounters unarmed ships that are out of their ZOC, the pirates can hijack the ships either by threatening them / hacking the ships. When the attack is out of the ZOC the AI doesn't know who is responsible. If a pirate is brazen enough to attack a ship in its ZOC, the AI declares war on the pirate. The pirates could sell the captured ships or upgrade them.
When civilizations fight eath other, a pirate can hijack an unprotected transport ship/colony and use it to conquer/colonize a world or trade for a colony ship / transport ship and upgrade either as needed for colonization or invasion. If a ship is unarmed, it wouldn't go throught the motion of a fight, it could surrender. The diplomatic skill of the pirates versus that of the other ship's captain would play into account in determining if he obtains a surrender or not.
When an AI civilization falls from fighting another civilization, the assets of that civilization join up the pirates. The pirates can hack into vacant starbases and sometimes acquire tech carelessly left on the computer systems. Pirates have two ways of sponsoring a shipyard. One is via a planet (which they can eventually obtain). The other is to accumulate spare parts from vanquished ships. It looks nice in the visual effect to have ships blow up all the time in combat, but in practice assuming kinetics and lasers should do limited damage to the ship. (Much as the airplanes of WWII could sometimes be repaired and brought back into action). Missles do a lot more damage but stilll much it could be towed into a starbase and repaired. It's not like those ships run on gasoline.
An AI could hire pirates to attack another AI in order to weaken it without fear of retaliation.
The minor races who sometimes get bullied by the major races could also help the pirates sometimes.
