I am sure a couple will not hurt, if you are talking 20 odd then..may be best to limit them but a couple should be good to go, just be sure to keep an eye out for dead or empty shells.
Make sure when you do your water changes to gravel vac everywhere and keep an eye out if you suck a shell up. If the shells are empty don't think there is much of a problem. Because mine aren't used to snails that much I have to scoop mine out if can see them as there are bits left in the shell. My snails don't live in the tank, I attach them to the glass and watch them "what is this?!" and have a go, it is great fun to watch
Ah. I'd be inclined to get something a touch bigger really as I know I can live in a boxroom but not sure I'd be completely happy It's your choice of course