cute little guys, where did you purchase them?
were they labeled as pygocentrus ternetzi or as gold piranhas?
gold piranha is usually known as serrasalmus maculatus, if that is the case then you will have to keep an eye on them as they might start to turn each other to pieces. cohab them is possible but not without risk.
best of luck and do hope they turn out to be pygocentrus ternetzi