well . . . . yes and no. They do not stay small, and in the process of growing can be very aggressive, even if they dont need the shell.
I have had mine for quite some time now, there are two left from an original dozen when the tank was set up about a yuear and a half ago. They started about the size of a pencil eraser and are now in a margarita snail shell and both have been for at least 6 months.
As for cleanining, they will not touch the hair algae, but they will eat coralline. In my tank this is a good thing because I have tons of it, and where they eat it, it just grows back in a new color, or darker shade. Its actually a nice benefit. They will also DEVOUR any uneaten food, dead or decaying matter, and detritus.
However the best part of these snails is their behavior. These guys are a RIOT to watch. They are so ridiculously fast! I watch them run all the way across the tank, racing towards a piece of food. Its hysterical . . . .
They are definitely my favorite hermits without a doubt.