When I started my tank I had gravel and a sally lightfoot and no snails. Later I added a couple of cleaner shrimp. I had some hair algae, but not too bad and the green stuff never attempted to take the whole tank, just a rock or two.
A year into reefkeeping I got rid of the gravel, and coincidently I had an enormous bloom of red hair algae. It was everywhere. I put 2 large "turbos" or were they "trochus" snails in the tank, and they ate it all! Worried that they would starve, I gave them to a friend with an algae problem and a larger tank. The red stuff came back, so I bought another snail. One large snail in my 55 gallon is keeping the hair and other algaes at bay, the tank is clean looking without having to worry that the snail would starve,...seems balanced.
One disadvantage, I would think, of having 100 snails in the tank, is how do you keep track of them. They can hide so well, you might wonder if some have died...
Anyway, I just thought I would add my 2 cents on what is working for me: 1 large snail. I love him.