FWIW, the FASA ship wouldn't result in overlap as such, because there was no FASA USS Ambassador. Rather, the class ship was one USS Ambassador Hardin, and the rest of the names were also in-jokes camouflaged as the names of politicians or other leading figures.
So far, we have every reason to believe that the class of which the Horatio was part had a ship named USS Ambassador as the class ship. Clearly the class isn't named thematically, as the known names are not ones of known ambassadors (Gandhi might fit with a bit of stretching, but not Horatio herself unless we postulate a fictional Ambassador Horatio, and Enterprise would certainly break the pattern). But we could always speculate that "Ambassador class" is Starfleet shorthand for the class launched by USS Ambassador Nixon, just like "New Orleans class" supposedly is shorthand for the class launched by USS City of New Orleans (yes, I know, just another in-joke in the Encyclopedia, but so many take it seriously)...
As for the general concept of overlapping classes, the Royal Navy has to struggle with that often enough: it always has a County class, a Town class, a Duke class, a Tribal class and a River class to juggle, and it's sometimes a very close call to get the last member of the previous County class stricken from strength before the next County class is introduced...
Timo Saloniemi