OC, I'm sorry. It's just, I don't want to get into her friend's business. They had to have bought the reed fish knowing (or at least finding out) how big they get. If they want mollies, I say let em have it.
I have kept huge carnivorous fish in the past, and they ate the occasional smaller fish in the tank. I never put the smaller fish in their tank on purpose, not because its 'cruel' for a big fish to eat a little fish, but because is very poor nutrition for the larger fish.
In all cases, the larger meat-eating fish grew faster than their smaller tankmakes and eventually picked off a few. When I saw what was going on, I moved the smaller fish to another tank.
You say that you don't want to get into SlateSpider's friend's business? But you answered the question of 'how many mollies will not overstock the tank,' so you are already in the friend's 'business.' SlateSpider didn't ask 'How many baby mollies should my friend feed her tank's fish?' No one that feeds live foods to carnivorous fish is worried about 'overstocking' the tank with the fish intended for food. You clearly did not originally think the mollies were being offered as food to the tank when you asked to make sure they were compatible first.
It seems to me that you answered 'about 5' without really knowing much about rope fish, or the size of the still unknown tetras and pleco. Without knowing the answers about the tetras and pleco, no way to answer how many would overstock the tank.