Watch out if you're going with a snail many (maybe most I can't remember) can multiply on their own and you will end up with THOUSANDS of snails, your tank will be clean, but who really wants a thousand snails in their little 5 gal?
I believe you're thinking of pond snails, which are generally regarded as pests by everyone. They are asexual, so they don't need a mate to reproduce. Mystery snails, apple snails, and nerite snails require a mate to reproduce, so if you only have one, you won't have any unwanted babies.
A pleco in a 10 gallon is not a good idea, just to note (I know the OP does not have a 10 gallon) Some people get away with corys, but IMO snails are much better suited for bettas.
Some pleco breeds stay small and get a max of 3" and those variety are fine in a 10 gal.
I would still be cautious with the plecos though... what breeds stay that little?
I respectfully disagree. IMO-the smaller plecos shouldn't be kept in anything smaller than a 20g. They produce a lot of waste.
try a bristle-nose catfish I have one in with my fighter at the moment and they are fine together. They are in a 5 gallen tank the moment and the catfish will eventually get to about 15cm but that will take over 5 years for him to get that big. I wouldnt recommend snails.