Realistically, as a Nebula class ship, the T'Kumbra likely has a crew of 900. I don't see how they could all be Vulcan. Even with Solok being a rascist prick, wouldn't he really only have a say in his senior staff? Surely the captain doesn't decide every individual crew member of a ship, just the senior staff and other key officer posts. The bulk of the enlisted crew would simply be assigned by Starfleet and Solok would have to make do. And really, I don't see Starfleet assigning only people of a certain species to a ship with a crew of 900.