Eyebiter is a bit of a squeeze, they aren’t as huge as others, but they are active, so a 75 is probably a better tank size for them.
Good puffers are a school of Amazon puffers or schoutodini puffers (both are somewhat peaceful and tolerant of tankmates), dragon puffers, arrowhead puffers, and hairy puffers, just to name a few.
Exodons generally should not be mixed with other fish. They are aggressive scale and fin eaters and will likely harass all of their tankmates. Only things with high success are large loaches or armored catfish. Look into congo tetras if you want to keep a bichir with them.
In nature, the biggest threat to red bellied piranhas are other piranhas, they do fine on their own. Solo reds will be a bit less active as they do not feel as secure, but once established will frequently patrol the tank. They are not as aggressive as you would think. They are voracious predators to smaller animals, but will only attack larger prey if starved (look up Roosevelt piranha myth). When I cleaned my tank it did the same thing every time: play dead underneath a log, it even let me pet it. They are extremely skittish, which I believe to be the cause of death for mine (I kept it with a banded leporinus, extremely aggressive, he must have nipped the piranha from behind overnight causing it to bolt straight into the wall, concussing and killing the piranha). You can attempt a school of dither fish with the piranha, just be prepared to replace them as the piranha likely eats them (if you have breeding guppies they would be a good source).
