Makes no sense. If Zangetsu is his Quincy power, then why does Zangetsu have Getsuga Tensho and Final Getsuga, techniques presumably passed down by his Shinigami father and thus would be part of his Shinigami lineage not his Quincy lineage?
You can argue that his Shinigami powers are a lot like a Quincy's, but that does nothing to explain why Isshin has them.
No, Quincies eradicate Hollows completely, destroying the soul.
Shinigamis cleanse the soul when they kill a Hollow, allowing it to go to Soul Society. However they can only cleanse a soul of sins committed while being a Hollow - any sins from mortal life can result in the Hollow being condemned to hell.
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