I don't think any self respect individual will claim himself/herself knows everything.
in regards to your question about cohab of different species of piranhas, some has done it before for a good period of time but in majority it didn't work out. I think mostly is because of the lack of space due to the small tank size we have and the setup of the tank. the other reason why such cohab will most likely not work is due to the natural behaviors of the fish. Most serrasalmus are ambush predators in order to do so they need to be unseen, so any fish you place within it's territory will either be food or potential competitors or so kind of threat, so out of instinct it'll either try to eat them or drive them away or hide from them. with being in a small glass cage as compare to a great river most likely it'll result in the death of one of the fish you place in the tank, sometimes both due to stress. the people that had success in the past usually had a relatively big tank (the one I know of had a 300g tank) and proper setup (hiding place for each fish) and just right number of fish (so the tank isn't over stocked and each have their own space). of course the personality of each fish is important as well.
can cohab work for sure? certainly, if conditions are right. however, most of us cannot provide such condition; and I believe statistically most of the cohab attempts ended in failure. Unless you are very experienced and have the money to burn, you shouldn't attempt such cohab. even if you do decide to do so, should keep a close eye on them and have a separate tank ready just in case they start to attack each other.