I've long felt that fleet captain was less an official rank & more an unofficial title to designate a senior captain, much as commodore is presently used in the USN.
I would agree except for the line that says Pike "was promoted to fleet captain." People tend to use that phrasing for promotions to substantive grades. In the US Navy example, people wouldn't say someone was "promoted to commodore," since they are actually a captain. They would say "appointed a commodore," "assigned as commodore" etc.
Consider how easily Picard was able to take overall command of both the tachyon detection fleet & the anti-Borg Earth defense fleet. Perhaps it was not simply notoriety or command orders, but a fact of rank;
There doesn't have to be a separate grade for that, though. Officers of the same rank are either senior or junior to each other, there are no ties. Picard could have taken that command on the basis of his seniority as a captain alone.
[...] he may "correctly" be Fleet Captain Picard, & due to not being a pompous ass, does not insist on usage of the full title.
There are generally standards of protocol for that kind of thing, and everyone of the same position is treated the same and subordinates don't have to worry about finding out a superior's personal preference. If Picard's full title was fleet captain, one would expect it to have come up at least once.
I figure that Fleet Captain is another way of saying "Senior Captain." An entirely separate rank immediately above that of Captain.
Agreed, at least that seems to have been the intent. Why Starfleet would need that grade is open for speculation, especially if it's between captain and commodore, as we saw a commodore function essentially the same as a captain.
I don't think Fleet Captain was a separate rank. It's just an honorific, nothing more. IIRC, the TOS writers took awhile before deciding on a straight-up Navy-style rank system; this is just an example of that.
The grades of captain, lieutenant commander and lieutenant were established within the first two episodes produced, so it seems like they were leaning toward a naval rank system very early on. Commodore, ensign and a junior grade of lieutenant had been established by the time "fleet captain" was used.