If you're talking about RBPs they will never reach 16" there is very few that get to 12" in captivity.
In reality 3 is a good number in a 55 but take into consideration that if one gets messed with and dies you'll end up with one. So what you need to do is get 7 of them. And get them small and as they grow you'll have extras that if you have dead loss it will be ok and then you'll be able to pick out the three you want once they get around 5-6" and get rid of the rest.
Oh and BTW no tankmates. It WILL die. And that will eat damn near anything that gets put into there tank. floating pellet or any type, krill, live, talapia, market shrimp. Whatever you got. But I wouldn't suggest steak. Read meat isn't the best for the digestive system of Ps. and they usually only eat red meat in the wild when it's the dry season and they get stuck in pools that run out of a food source. And then they will eat anything to survive.