Mr. Laser Beam wrote:
In Captain Solok's case, it's understandable, as his evident racial prejudice against Humans would almost certainly preclude him serving on the same ship as them. He believes Vulcans are a superior race (which is in itself an anti-Federation and illogical view), so naturally he would want only Vulcans on his crew.
That's your own opinion speaking. If Solok truly thought Humans were an inferior species, then it is illogical for him to be associated in any kind with them (and that includes both Federation citizenship and service in Starfleet).
It's also possible ships are primarily staffed by the species indigenous to the system where that ship was constructed. With about 150 planets, there must be more major shipyards than San Francisco and Utopia Planitia.
This right here. The T'Kumbra
probably was launched from Seleya Station or from some other facility in the Vulcan System.
There probably are a few ships launched from the Sol System that may have all-Human crews.
On the other hand, you have the Starship Hera
that had a Human captain but a mostly Vulcan crew, so starship crews can be either multi-species or single-species, with the mix probably determined mostly by where the ship is deployed from and what personnel are available at the time, IMO.