13" is by no means wide enough to keep red belly piranhas for life. It would work with either the serrasalmus irratons or sanchezi, but not any of the pygos for the long run. I know that you said that you were only planning on keeping them temporarily, from the sounds of it you are setting yourself up for disappointment. Serra's are skittish fish. Red bellies are incredibly skittish when not kept in larger groups. I believe I read one instance of a single red belly in a tank to himself taking 4 years to come out of his shell. Not to mention that you have more then one in this small tank being kept without food for multiple weeks? Chances are high that you will lose some. Try it if you want, the tank is too small but its not like its a 5 gallon tank, but I want you to prepare yourself for a let down if you do not plan on keeping them for the long run. It takes these fish a long time, alot of times, to get used to their keepers. From the sounds of it, you are not planning putting in that time.