Sorry :( Fish store employees tend to be pretty ignorant, unfortunately. No stress as long as you acclimate. My fish are more stressed by being confined to the the cup than the new water.
That'd be a good idea with the snail especially since you can't test ammonia right now. Also using Prime as your conditioner will render some of the ammonia harmless by converting it to ammonium. That's also helpful when keeping smaller uncycled tanks.