I would think that a turtle could eat a piranha depending on the size differential. They are also pretty messy. If you have space for a turtle, why not just get more piranhas. Also, a standard 55 will eventually be too narrow for piranhas.
If you try to put in crayfish I'd suggest lots of hiding spots that the p's can't get into so that when they molt there is a place to hide. I don't know the size of the crayfish but if they are large they might go after the piranhas.