Space Monsters!!!

[Suicide Junkie]

RE cloaking:
Translation:
- The spacemonsters lair can't ever be seen or attacked.
- The only way to kill the infested planets is to blow up the star.

The high-tech beastie thing is probably not scalable... At the beginning, they will be too powerful, and at endgame they will be only a nuisance.
The swarm could probably be handled better, since as the monsters take over new territory, their swarms will smoothly scale up. This is also probably good against minefields, since the loss of a hundred would not hurt the overall swarm later on in the game.

I would go with the swarm preferably, since the AI can screw up tactically with a few ships and not ruin the whole attack.

For variety, of course, there will have to be plenty of Borg monsters and middle of the road evil.