In my current game I've seen plenty of large AI fleets. Too bad they were mostly Cue Cappa. I had a nice time using a single ship captured via Crew Insurrection to capture the two front ships in the fleet, then use the Alleigence Converters on them to convert the next two closest ships, then used those to capture... on and on. I had a single captured dreadnaught capture a fleet of 20 dreadnaughts. That is sad
Other than that, the AI has been kinda sorta ok with sending fleets at me (still can't seem to figure out how to send large fleets through a warp point to kill my lone fighter group there... but that was over 50 turns ago