I had a single one for a while, then I had a pair of them. They never ate snails. That's just my experience. If you're looking for snail control, don't count on it.
In terms of the plants, I should qualify that. They didn't dig up established plants. However, you probably won't be able to do a fine substrate planting with those guys around, because they will uproot anything that's not firmly in the substrate or is only held down by a few pieces of gravel.
I'd really advise against overstocking. While it may seem like it's OK because you have enough filtration or do a lot of water changes, if something happens like a vacation, sudden family emergency or a power outage, you're much more likely to lose everything. Also bear in mind that a 55g has rather limited bottom feeder space, and you have a lot of bottom feeders.