As for the original premise — a town of as small as 100 can support a pretty solid fleet size. There might be the need in such a place to “plan” — reserve a car a bit ahead of time, but that’s true of many things in such towns, the grocery isn’t 24 hours and the like.
If the self driving car model becomes the primary model in urban areas, the economy of scale for cars that have steering wheels and accellerators goes away, making those vehicles more expensive and driving the model in smaller towns.
A town of 100 right now probably has 250 cars or so, the cost of which is fully borne by the owner users. Perhaps it will be more expensive to hail a car in a town of 100, but it will still be cheaper than owning a car is now. And my wife’s town of 200 is a few miles away from several other cars of that size, a fleet vehicle isn’t tied to any one location, if you take one car to Chicago, you could easily be taking a completely different vehicle back, or to a 3rd location where you get another car to take you home. Since the car is not tied to a person owner (current Uber) they can go anywhere.
If you live on a ranch in outer Montana, well, there are plenty of legacy vehicles for you to use. Good luck finding a gas station however.