sorry to disagree but mystery snails in an unfiltered 1 gal is a recipe for trouble.
honestly with 2 you may get away with it with frequent water changes but you really want to start with at least 6 and mystery snails do accutuly put out here shair of waste and have been known to leave tanks with poor water quality and die on the floor.
also you have to consider the fact that mystery snails need a mnimum of 3" (but 6" is recomended) air gap because they lay there eggs above the water line but they are very clever as they like to place them in a spot where it will be humid enough but not too much so as to wreck the eggs so the 1 gal tank will have considerably less then one gallon of water in it which between uneaten food and waste build up will result in water quality issuses.
in addition and substrate, plants etc will also reduce the amount of water in the tank but would probably be a good idea to try to develon some form of bactirial filtration, inefficiant as it would be in this situation
