well rwhayley. That tank is much to small for all those fish combined and if they all do fine now it wont last long. one or all of those fish will be lunch with only one left.
monstermatt-we put a pleco that was about the same size as the piranahs and gave it a hiding place. Its still living and it does come out ....
Snails. we also purchased a large snail. it lived for a while before they smashed the shell and ate it. We are looking at purchasing one with the spikes..
maybe some smaller fish, but once they get big enough to be worth eating they will.
We are actually shocked that the pleco has lived for a year with no harm done to him.
You could try some others but make sure there are lots of hiding places... so if the piranahs do try to eat them they have a chance...
good luck