I wouldn't get a mystery snail. They get large, they poop like crazy, they do zilch for algae, and they stink to high heaven if they die. Seriously, it is the single worst thing I have ever smelt, and it sticks to your hands for days. You are nicely stocked as is - don't go adding snails.
Some things that would help your neons un-stress:
- More cover. Open spaces make fish feel vulnerable to predators. If you add in more plants (I would choose some easy-to-keep live ones like java fern, anubias, hornwort, cabomba, ludwigia, rotala or wisteria), they will feel safer. Floating plants are also great for this.
- Cover the background. A plain black background would not only make their colours seem more intense, it would make them feel safer, as nothing can see through the back of the tank.
- Black substrate. Also makes their colours pop, and seems to make some fish feel more secure.
I hope this helps. :)