Originally Posted by horatio
It is just a money thing, that's why there were more aliens aboard the E in TAS and more aliens in subsequent series like DS9.
Starfleet HQ should not be on Earth, humans should be but a tiny fraction of Starfleet personal, but it is just not affordable and a certain "human centricity" is hard to take out of Trek. Although it is TV and cinema stuff, we can still let our phantasy flow and imagine more aliens in Starfleet.
I honestly think it had less to do with budget constrains then fear the audience could feel alienated, literally. The heroes had to represent America in contrast to foreign challenges and threads. Thats why not only Starfleet but also the Office of the President of the Federation had to be located on Earth and both had to be located in the U.S. (where else
The only exeption was DS9. I dont know how many Episodes there are of that show where all main characters in the main plot are aliens.
Concerning the idea of "seperatism" in Starfleet: Lets assume the United Federation is less a USA in space and more like the United Nations, the European Union or NATO. Every world would have its own Starfleet but in regard of making cooperation more efficient rules and regulations are relatively equal as equipment and ships too.
So the Yorktown could be a Constellation Class owned by the Vulcan gouvernment (as the french NATO partner owns american Hercules airplanes for instance).