To be fair, Kingler is not a Hermit crab, so while he does indeed have a shell it most likely grows as a part of him(like real crabs), kind of like our bones. So smashing it would likely kill him.
All crabs shed their shells throughout their life, it is called molting. I see no reason why Kingler should not learn SS, but I see no reason that it should either.
Now as far as the new guy goes, meh it is just meh. I mean I don't hate it, but I don't care for it. I would have rather had Honedge just have been a only stage.