Gerbils aren't terribly social animals, and finding vet care for small mammals can be tricky in some areas.

As for a fish or a snake, I wouldn't trust the average person to take care of either a fish OR a snake. Most people think you can just plop those in a bowl and go on with your life, and that's not really right. The next person to tell me that goldfish live short lives anyway is gonna get an earful.