You can already order fleets to join other fleets as sub-fleets. I don't think there is a 'Detach Sub-Fleet' order (rather than a button), but that could be added.
Thank you for considering it! I've made my case on the usefulness of more sub-fleet, squadron and escort-related movement orders here
https://aurora2.pentarch.org/index.php?topic=13681.0, sorry if it ended up being too much of a wall of text, but what it boils down to is that the more types of fleet orders can be executed as movement orders (which can be cycled or queued in advance), instead of only being available as dedicated buttons or through menu dragging (which can't), the further ahead we'll be able to plan and have our plans be executed seamlessly without the need for further adjustments.
There's a plethora of use cases for this: convoy escort rotation, jump tender handling, damaged parasite recovery, parasite transport between colonies, it's all there.
As for the FC assignment, it only gets really time-consuming in those rare massive battles where one has numerous priority targets and too much firepower to focus fire on each and every one in turn. I''ll admit right-click targeting will be of little use there
though if the drop-down list would separate into class-based sub-menus past a certain size, I wouldn't be so sure, that's why I'm advocating for a class priority system for weighted selection during auto-targeting instead.