But you also have to take into account that Starfleet is not a true extension of today's navies and really doesn't have to do everything the same way-especially after 300 or 400 years. If anything, the real distinction between Starfleet and today's navies is how different it's chain of command and promotion policies are compared to the way things are today.
But unless they travel forward in time to see how every officer does in higher command, the best way to get the best commanders is to give them a position of responsibility, see how they do, promote the best, give them more responsibility, see how they do and so on. Given the nature of Starfleet we've seen, which seems to have a fairly traditional military pyramid, I think it's realistic that they would have a promotion system pretty similar to what we have today.
However, we know that there's a position of "Starfleet Chief of Staff." Would that just be another title for the five piped "Commander-in-Chief"?
The trend since WW2 has been to divide the top levels of command into what we might call, for lack of better terms, "administrative" and "operational." The US Joint Chiefs of Staff, though the top officers of their services, are not in the chain of command for operational units "in the field." They are in charge of the big picture, long-term planning and policy to keep their services ready to do what is required. The top operational commanders are the Unified Combatant Commanders, who command forces of all the services in their areas. They used to be CinC's as Timo mentioned, until George W. Bush decided that title would be for himself only.
I tend to think that the Starfleet Chief of Staff is like the CNO, top officer of the service and top adviser to the president, but not directly in charge of the operating fleet(s). That would be where the CinCs come in.
Also, in that system, what would be the one-star Admirals'/Commodores' responsiblities?
Probably a broad spectrum, from command of major bases and small squadrons to all sorts of staff jobs.
the navy still uses the title of commodore for its DESRON commanding officers, but they're usually full birds, not one stars. probably more of a ceremonial title. maybe i should've asked old pumpkin head about that (one of the commodores i served under)
There are a number of O-6 billets (COMDESRON, COMSUBRON, COMPHIBRON &c) that entitle the holder to a flag called a "broad command pennant" the same way a flag officer has a personal rank flag. The title of commodore goes with the pennant.
--Justin