Yes, I like your point about different builders throughout the Federation while building to specs may vary sligtly. If you ever downloaded my little SS Aurora, I made a Vulcan verison as well as a Fed version, with different styles in mind reflecting different places they were constructed at.
Also, the reference to only a dozen Connies was the TOS episode "Tomorrow is Yesterday" wher ethe ship travels back to the 1960's by accident and they pick up that airforce pilot. Kirk mentions "There are only a dozen like it in the fleet". Franz Joseph got around that by having the 1701 be a part of the Constitution class but there also being a better version called the BonHomme Richard class, of which dozens were built, and the USS Defiant that disappears in "The Tholian Web" was one of these.