Perhaps I am mistaken, but as far as I am aware it is impossible to fully disband a fleet, or to at least merge them. This becomes a severe issue when enemy systems are limited to only 2 attacking fleets, and you have many scattered forces. Additionally, I don't want to have to be carrying around terrible fleets such as the space marines as a whole, or fleets consisting of only light cruisers and escorts.
The ability to disband and merge said fleets would be much appreciated, and if already included, it would be nice to see it clarified.