I think when it comes to assessing versimillitude in a game, we gotta ask ourselves 'how did i feel when i very first encountered this aspect of the game' (in this case, the shi)
Well, when i first encountered them, it jarred me. I was enjoying ploughing through the awesome post-apocalyptic retro-future western style world, and most everything just 'seemed to fit'. Kind of like the oracle says in 'Matrix reloaded' - 'the programs that are doing what they're supposed to do you don't even notice'
As soon as i encountered the shi i felt like something was wrong, and the beautiful illusion the game was weaving for me was cracked a little.
Now, i could jump into the intellectual break-down of if the crew of a submarine has enough crew to start a town, how many people are required to make gene-pool healthy, what kind of society they would make etc. And good points they are.
But for me, the most important thing is how these 'added ingredients' taste. If they taste good they make me want to keep on eating. IMO, when BIS sprinkled in some 'gangland' mafia stuff in the form of New Reno, i lapped it up cos it tasted great and went with the rest of the meal. But they threw in a dash of Bruce-lee, kung fu, eastern culture it tasted bitter, like too much lemon juice, and put me off my food a little ....