1. You can migrate class by class or even method by method. 2. You add type support to a method by prefixing it with a special lambda (Sig) that is read at class evaluation. 3. It provides its benefits right away. But really, most value comes from network effects. You won't get type warnings if the method you're calling isnt typed, because the return type cannot then be inferred. 4. No gems are typed right now. We type out type definitions for external dependencies in *.rbi files.
It's funny that you mention typescript because in that sense the two are quite similar. Types are defined either inline or in separate files .rbi/types.d
Opinions in the company are split probably roughly ~60/40 if Sorbet is worth the effort or not with support having a smallish majority.