In the old master of orion II there was boarding!

It was a nice option to capture secret technology in battle. You could install anti-boarding modules, to prevent it too.

I would guess this could be a viable income method for pirates:

"Arrr ... we got ya nice shiny colony ship .. but you can have it back .. for a reasonable price of course! But that frigate we will keep. Makes a good addition to my fleet! - Admiral red beard out!"

Would make Pirates a viable 12th Race. That'd be fun to see.

hmm .. some more or less nomadic space pirates looking for plunder. That would be a fun idea.

They can colonize asteroids and Gas giants with moons.

Maybe not stronger per se, but they should get stealthy. Cloaking tech, hidden bases, even mobile bases.

A pirate don't flag the jolly roger on first sight. They flag something unsuspicoius.Only when they get close enough, they typically flagged their own banner to intimidate the victim. So in this game it would only make sense, when a pirate disguises as an allied or friendly AI ship.

Pirates are specialist in robbing and plundering. So they should look out for a frighter or utility ship. No pirate in their right mind, would attack a real warship. They were fast enough to run away from anything scary but did pack enough punch, to take on a well armed merchant. Oh, where have you found the setting for the difficulty?