It would be very hard to breed anything in a 5 gallon tank. But if you want to try it either go with guppies, which I know some about, or maybe ghost shrimp, which I dont know anything about.
If you did guppies, you would only want 1 male and 2 females. You are going to have to do water changes every couple days once you get babies. In a tank that small it is going to be very difficult. Personally, if you want to breed anything get another tank, get a 10 gallon. With the 5 gallon tank and a 10 you can breed guppies much easier.