My experience is with C#, but I think I can answer your questions. First, Fakes can stub anything that can be implemented or extended.
Stubs and Shims work by creating a public property of type Action or Func with the correct arguments and return type, and overrides methods to execute said delegate. You can therefore pass in any matching delegate, as a MethodGroup or as a lambda or traditional delegate.
Judging from the documentation, you are syntactically correct. How are you using the stub? Stubs are only one instance. If your goal is to override ClustersGet on any plan that appears, rather than an instance you know will be used, consider using shims or redesigning your method to support dependency injection, namely passing it the Plan object.