goldfish are interesting and you have to be careful because they will eat until they die.. literally..im sure they will be fine with a pinch of flake 2-3 times a day.. the reason they poke around looking for food is natural, when they get bigger ( and trust me they will, generally growing to the size of the tank ) they will pick up gravel in their mouth and spit it back out .. taking the food out from the gravel..
your question on snails.. if the snail is bigger then the mouth of the goldfish theres absolutly no way of it eating it.. although i dont think the goldfish would attempt to eat a snail anyway although they might.. BUT .. i should warn you, snails are hermaphadites ( except i believe a few species ) and thus making them asexual. this means one snail soon leads to 2 then 3 then 8 and so on and so on. i wouldnt be discouraged though because snails are great at cleaning up but just keep their reproductive nature in mind. happy fishing