I don't have fleets, in general. Why do I need them?
To group ships and make them act together.
I always have the main attack fleet, consisting of various frigates, cruisers and capital ships, a carrier group consisting of carrier capital and carrier cruisers (set to not auto-attack, they sit on the edge of the gravity well and let their strikecraft do the fighting),a demolition crew consisting of anti-structure cruisers, and planet killers consisting of bombing frigates.