Please explain the use of the rank of Fleet Captain (distinctly British) in several episodes, then...
Sto lat'
Riik
Actually, it's not a
Rank in the UK. It's a
posting. The rank is Captain either way. And it was used in the USN as well... but only rarely, as only Civil War and later was there a rank above Captain in the USN.
Likewise, Commodore was a Rank in StarFleet. Commodore in the US was technically a posting, but treated in most ways as a rank, during the age of sail. Then again, the US didn't have Admirals until shortly before the USCW... We see commodores in Star Fleet... And they're serving as captains with additional duty and insignia... like Stocker and Tracy... or Base commanders ... as Mendez... a very US age-of-sail use.
Then again, starfleet lacks the distinctions of Post-Captain, Ship's Captain, Flag Captain.
Noting that a Fleet Captain is used alternately in history for a rank of captain serving as a Fleet's 1st Officer, and for a Captain Commanding a Fleet but not entitled to a Commodore Title (in the USN, for lack of seniority or letters authorizing).
Flag Captain is not a separate title in the US, and unlike the UK, not noted as a position, since the Flagships weren't as tightly associated with the flag officers (and until WW I, the commodores were often the ship's captain, anyway). And unlike the UK, it's not used in Starfleet, either.
Post-Captain isn't used, either, in Starfleet. It's the lovely UK term for a captain not in a command slot.
In fact, we don't know what a Starfleet Fleet Captain is during TOS, but we do know what it is in TNG era - the head of each ship class's design/maintenance/Upgrade team. Very different from the UK use. During TNG, we meet two of them - both retired medically. Perhaps it's just the title for a retired commodore... perhaps it's a captain commanding a squadron, since commodore is a rank in Starfleet.
US Commodores pre-Civil War wore captain's shoulder boards with a star on one of them. Civil war and on, an extra stripe over a captain; grouped 3-1-3.