Mine doesn't bother small snails or hermits, its mainly shrimp you have to worry about. The longer they're in the tank (without other fish) the more agro they get, 1 month isn't too bad but much longer and they might not tolerate any new introductions. Also the size of the sixline can be a factor, if its smaller than the other fish it might not bother them at all, and the larger the tank the better.
Not sure about compatibility with other wrasses. As with any fish not all fish of the same species have the same personality, but generally sixlines get territorial.