hey!
you have to sppecify what pirahna ur talkin bout, red belly? rhom?
at a small age for a rhom then ur rhom would probably get owned until its teeth develop. wut size cons. i have seen i think it was unbeatble c or sumbody (it was a teenager guy) with a rhom and a jak dempsey the same size. it look like they were fine except the rhom was in the corner, and the dempsies fin's were allitle damaged.
i have seen some successful tanks with a single red-bellied pirahna with cichlids and catfish. the pirahna got chased alittle but he was okay???
if ur gonna do so make sure u dont get one more than 2 red-bellied pirahna wait jus get one. becuz u know wut will happen ur cichlids will get wasted.