My experience (on 7.0/7.0, v4.054) has been similar. After watching my diverse fleet get trounced repeatedly by 15-24 marauder ships, I found a new strategy:
Abandon the planet, let the marauders torch everything, and then rebuild. I had force fields around the key command centers (AI was using teleporting ships, so force fields served duel usage), and once the marauders split up, they flew relatively harmlessly through the rest of my space. Once I started getting fleets of 700 ships, I could usually take out the marauders without too many losses, but they'd still carve up an AI planet in an impressive fashion. If I could just figure out how they pick their targets, there are some more AI planets I'd like to see torn apart by them. (The AI seems to respond poorly to them because of it's activation logic; their frigates often stay at a distance and just destroy once ship or guard post after another.)