How much covering do you have? If you got plenty of plants and cover you could do a lot of different things with less risk. I know personally I've had massive luck with convicts but they nip the piranha and my 2 yoyo loaches seem to always be content. The pleco though, he lasted less than 24 hours. Figured out that wasn't going to be such a great idea when he had 5 massive body guards when I put the pleco in the bottom... I woke up and found a munch trail in the alge and a single scale the chiclids were fighting over. 