You should get a conch. I love mine. I think they are neat to watch. As far as the snail additions, I would just watch the tank and see how it goes and if you start seeing algae build in a certain area of the tank, then buy snails that will work on that area. I don't buy into the 1 snail per gallon of water hype. I hate crabs, but if you insist on emeralds just do a pair. Cleaner shrimp are neat to watch.