I have an idea, what about another spoiler enemy, space zombies?
They'd be like the swarm, only purely reliant on anything else there. The swarm can reproduce via wrecks that workers can create after mining, the zombies are totally reliant on other races to expand their fleet. They would have some motherships filled with small "troop transports" filled with "zombies" (which I picture more as tough organic matter rather than ships, the "zombies" being more like animalistic parasite things instead of dead sentients) and when a ship came in range they dumped out a few of the transports. The transports would then close in and ram straight through your hull, no need for the boarders to blast a hole, they would just punch right through your shields/armor. Then boarding combat would begin, when (if) the crew dies off the ship starts to badly contort and metal and flesh would fuse together to form this perverted organic body thing, a la Space Pirates and Zombies. These "zombie ships" would be armed with the weapons the ship had originally. If missiles were on board, they possibly could get turned into these organic suicide cannons which loaded small amounts of boarders (Say, 5 every missile size) onto a ship. That, combined with large enough/enough missiles could possibly land a whole company on board an enemy ship and the cycle would continue. If it could be added, these ships, being organic masses mixed with metal rather than solid metal, could regenerate damage at a slow rate. For the sake of balance, certain systems on the ship could be destroyed by the infection, like microwaves and shields. The marines/parasitic zombies of these vessels could have their readiness/strength increased by picking up life pods, where they would butcher the crew and add to their ranks. Obviously, this is a very rough idea and may need to be changed a lot to fit the current limitations of the game or to stop them from being able to run over everything they meet.