You don't need that many snails unless you plan to keep a nutrient-rich, low flow tank. My algae maintenance in a 125 is handled by 5 mexican turbos. That's it. If you avoid excess nitrate or phosphate (which feeds algae), and keep the flow levels up, a mass of snails will have a hard time finding food. Sand / detritus maintenance is done by 15 ceriths in my tank -- it's nice to have a little bit of sand stirring on the surface.
Whatever a snail eats a snail poops, minus the energy it takes to grow them and move them around. Adding a bunch of snails means you'll need, well, a bunch of snails to keep the algae from the waste production down. Good water quality means less algae, and less need for snails.
Yes I have a few crabs left from the old tank, but they won't be replaced once they kick off -- snails do a better job on algae and crabs mostly just eat old food and snails anyway. If you must get crabs, I suggest the red legged hermits -- the only ones I've witnessed eating hair algae, and much safer for the tank than fully grown emerald crabs (which I've also witnessed eating hair algae).
I'd never have anywhere close to 1 snail per gallon in my system - I'd make skimmer / refugium / phosphate reactor / nitrate reactor / whatever upgrades first to combat the problem, not the symptoms...