I think you have the subject is proper prospective Cannuck. Most times when we use "Newfies", "Scotsmen", "Ukranians", it's just an example we take out of the blue to tell a joke- could just as easily be Martians.
Oh come on. If you use Martians then you won't get the response you're looking for. Newfies, Scotsmen or Ukranians are buried deep in your mind now and are always there for your wisecracks whenever the need arises. It doesn't come out of the blue. Just because it's been the norm up to this point does not make it right.