Hoppe's No. 9 has very little ammonia in it, so it may be fine with all nickel plated firearms, but maybe not soak them in Hoppe's No. 9, just use it to clean and then wipe away all the No.9 before oiling or waxing. That being said, the issue of ammonia and nickle plated firearms comes up every month or two. The issue is in how the nickel plating is applied.
If a firearm is first plated with copper, then nickel, if there is any break in that nickel finish, then ammonia will come into contact with and begin to dissolve the copper plating, then the nickel begins to lift and flake. S&W did not use a copper intermediate layer with their nickel finishes, they plated directly to the underlying steel or aluminum, so there is no issue with ammonia and factory nickel plated S&W firearms.