My understanding is that the most heavily discouraged scenario is defining a method where you “own” neither the generic function nor the type. So if dimension is defined in your package, this should not be problematic.
Regarding step! (ie extending a function with methods that have a different, previously non-existent signature), I would still be cautious. If you are the only one doing it, it should be fine and not interfere, but if multiple packages decide to do it, you can easily run into the original problem.