I personally tend to like the "big fleet" concept that seems more prevalent in some of the FASA and fan-produced technical sources, rather than going solely by canon.
I am with you on the big fleet idea. Unless there is some limiting factor we don't know about (anti matter production, dilithium crystal availability, etc.) I assume a civilization 50 - 150 member world's should have ample personnel to man 100,000s of ships.
I tend to view Starfleet as primarily being the Federation's military force, with both a small number of dedicated warships and a much larger segment of vessels that can be multipurpose or geared for exploration.
Starfleet seems to be a combined sea, air, land, space armed force that has absorbed the functions of NASA and NOAA in addition to the military forces.
f we go towards the UFP as being more analogous to the UN rather than the U.S., then I think perhaps it's plausible that member worlds like Vulcan and Tellar might retain their own smaller local fleets
I think it's a more federal union than the UN or even the EU. Maybe somewhere between that and the US. There may still be planetary armed and police forces, in addition to scientific and academic institutions that might have their own ships.
FASA had mentioned elements like the "Blue Fleet" that were crewed mainly by Andorian personnel, but I'll have to check and see if they make any reference to localized fleets that weren't necessarily the same thing as Starfleet.
Even within Starfleet there would likely be variations in design and conditions that are species specific or tailored to a narrow range of species.
For instance, Andorians might prefer temperature and other atmospheric settings that would be uncomfortable for certain other species. Much colder for instance than humans would prefer. Vulcans might prefer it too hot for Andorians or humans.
Still other species may require atmospheric pressures and gravity that are too high for many species. There may even be Federation members from planets around red dwarfs who evolved to see in the infrared spectrum, or species with low temperature or non carbon based biochemistries. Ammonia, silicon, sulfur, etc.
Even with compatible living conditions, some species may average heights greater than wookies and some the size of Yoda. The ergonomics of panels, workstations, control interfaces, chairs, door heights, etc may need some customization.
One question I'm interested in has to do with refits and/or decommissionings.
I can see technology transfers such as occur in our own world. Ships and aircraft convertible to other purposes after some life extending refit and conversion.