Fate, I'm not trying to pile on here but that's simply not true.
Piranha's have probably the greatest healing powers in the fish world when it comes to physical trauma (I've seen piranhas with their stomach bit in half and their innards literally hanging out underneath survive and do great) but they can get sick like any other fish.
I'd say a minimum of 75g for 3 and 4 rbp's. Piranha's, especially Serra's but pygo's also, are extremely territorial. Even more so than cichlids, ime.
You can try it against our advice but they'll kill however many they need to feel comfortable and have their territory.