Honestly most aquatic turtles when you put them with fish you run a high risk of them eating your fish even if you have calm fish. Many people would say no aquatic turtle with any fish will be a long lived cohab. But since you have piranhas, there's no way anything will end up living in longevity in that tank. Either the piranhas will get eaten or more probable your piranhas will attack the turtle.
IMO ditch the piranhas if you want something that can have tankmates/or possibly setup a seperate reptile tank if that would help add variety.
I've never kept P's but aren't they a very skiddish fish? I would think an active turtle would drive your fish nuts. You might be able to keep a FRT with them but they are too rare and expensive to keep with P's. Plus, most aquatic turtle species would chase down P's in a heartbeat and eat them.
a guy a know tried it for awhile (turtle with P's). that didnt work out well at all. the P's started nipping at the turt's legs and hurting him. the turt would def be the one getting hurt in this equation.