If it's of any use, I must say that I have always made a distinction between fleet and squadron.
It's not an easy designation and seems to depend allot on who is saying something and where they are saying it. Also it has allot to do with numbers and class of ships, how highly ranked the commander is and what his mission comprises. But officially the Royal Navy in 1800 had 768 ships in service [Robinson]
This number was officially broken down into one large Fleet made up of 3 large squadrons, which at first had been small but by the 18th century had to be commanded by admirals. These squadrons were colour coded, the Red, the White and the Blue, each had three admirals commanding them, Admiral, Vice and Rear.
That's the official line anyway, and goes some way to explaining why the words were interchangeable to a degree. Yet at the same time a small group of warships with a propoderance of Frigates as opposed to ships of the line undertaking a specific mission would be more properly a squadron, even though it might sometimes be called a Fleet.
Fleets also operated on "Stations". And so Fleet and indeed squadron can also be an administrative and geographic term as well as a strategic one. I believe squadron commanders answered still to their Fleet commanders. All the way up to the lords of the admiralty.
Another thing to keep in mind is the varied responsibilities officers in the RN had by comparison to the army. Even frigate captains working independently were had levels of responsibility commensurate with colonels and on their own achieve large coups at sea and on land. When the officer is a commodore, an officer put in charge of more than one ship, the possibilities for independent action almost seem to double. Every sea officer from the master and commander of a single brig/sloop to a post captain on a frigate to a squadron commander could be seen to be a general or admiral in his own right.
So I suppose I'm saying that there was an official designation of what a squadron and fleet was, as well as a common one that could vary.