I would think that the dwarf gourami would pick on the sparkling gourami, especially with a tank as small as a 5 gallon. Dwarf gourami tend to be rather territorial.
Maybe some pygmy cories?
I'm not that I would add anything with a dwarf gourami and a mystery snail in a 5 gallon. I would do about 2.5 gallons for each of those since I'm assuming that by mystery snail you mean a Pomacea bridgesii, a type of apple snail. That's me though.
If it were a slightly larger tank I would suggest a school of pygmy cories, or even 2 female dwarf gourami if you can get your hands on them. Dwarf gourami really show interesting behavior when they are with their own kind.