I don't think it is possible to have too many shrimp. :) It is lovely to see cherry shrimp all over the place, and since they have such a tiny bioload it doesn't really matter if they breed like crazy, especially in a planted tank.
As for snails, it really depends on the type. Wash your plants carefully to avoid hitchhikers, and don't buy anything too prolific, such as ramshorns or MTS (unless you want them). Nerites (for algae) and assasin snails (for keeping snail populations under control) may be good. :)