A "pirate fleet" played a significant role in a number of the books, including IIRC in the Thrawn trilogy by Zahn and the Mara Jade stories.
This doesn't mean that they have something that amounts to a Capital ship ...
... BUT we don't really need one, do we? All we need is something that has attacks and specials and that can't be damaged.
So get a Pirate Fleet commander who operates from a cluster of smaller ships and fighters. The pirates get their invulnerability from their numbers and the disposability/ replaceability of their small ships. With fast small ships, they can remain at a safe distance, then send in a very fast raiding party to aid a particular squadron when called (i.e. when the Pirate Fleet turn meter bar is full).
Fire and Forget (Basic): The pirate fleet launches a salvo of missiles to aid their comrades while remaining at a safe distance. Targets of this attack have a +10% chance to evade. This attack does physical damage with large damage variance. If this attack fails to do damage to the target, remove 100% Turn Meter.
Raiding Party (Special): A small batch of fighters streaks in from hyperspace with escape vectors pre-plotted. The close jump confuses sensors and gains surprise, during which the pirates fire at random targets before hyper-jumping away again. Dispel Target Lock from all allies then do special damage to random targets 4 times. This attack cannot be evaded.
Jammer Pulse (Special): Dispel all buffs on enemies, with a 35% chance to Target Lock each enemy. Any enemy that had no buffs dispelled loses 100% turn meter and gains OffenseDOWN for 2 turns.
Juryrigged Systems, AKA "It's not my fault. It's not my fault!" (Unique): Pirates are adept at finding unconventional uses for otherwise common equipment. These frequently have the effect of increasing power and potency beyond what is normal for most ships, though they are sometimes subject to failure. All allied ships gain +50% potency and +30% offense, but immediately before each ally's turn (except for Pirate Fleet Command itself), there is a 10% chance to lose 20% TM.