they will eventually be eaten. theres alot of things that contribute to the likeliness or the chances of survival. such as the size of the piranhas, their attitude, the size of the tank, the feedings of the piranhas, decors to hide in/under, speed of other fishes, etc...
ive tried exodons, plecos, danios, tetras, and mollies. the ones that lasted a while were the plecos and the tetras. the pleco hardly moved and was big and lasted for almost a year, and the last tetra survived for about 4 months, could of been longer if i had more decors.