1 gallon is ok as long as you keep up with the water changes. The ammonia from their poo builds up very quickly in such a small space (think of swimming in your toilet bowl and not flushing!) so you need to do 100% water changes every other day ideally. At least every 2 or 3 days. That means 100% new water, rinse everything out with hot water including the tank, decorations, gravel, then add new conditioned water. Having a larger tank gives you the opportunity to cycle it, which means less water changes, and no 100% changes. So it is up to you, but just make sure you provide the proper upkeep for the size tank that you have.