I have MTS in just about all my tanks(especially the sand ones) and they do a great job of keeping the substrate aereated. Haven't noticed them eating plants, but if a leaf is dying they will eat that.
I would add only one if it were me. Snails reproduce like crazy laying hundreds of eggs....usually out of the water.....It's also important to remember that if you have any sort of loaches in your tank that they will more than likely eat the snails.
I just inherited snails in a plant that I just got. They're less than the size of a ladybug...I'm fishing them out and bringing them back to the store......
My son wants to keep them and name them - "Gary", from Spongebob of course - and "Larry" - the one that replaced Gary when he left Spongebob for Patrick!! LOL!!!
I haven't figured out a way to tell them they've got to go.
I'm not too sure about varieties, all I know is that they can multiply in a hurry..... And if your tank has enough food for them it might work or they will turn to whatever is available......like plants......