You're at max capacity for piranhas in that tank. if you upgraded to a 55-60, you could squeeze a couple more in. You could go with some tetra or silver dollars as dithers, other fish movement (besides feeders) might get your fish to "come out of it's shell"
funny you say that my freind has 2 4inch silver dollars he lives next door, wont they freak out when they see the piranha though??