I have definitely seen folks use the id created by the auth system but nothing else. Everything else keys off the GUID, which should be immutable, and is stored in application specific tables.

The password and username are in the auth system, but everything else stays in control of the application, in one database.

I don't see why you couldn't use that pattern no matter which external auth provider you are using. You'd just give up things like self service registration, but that may be the right choice for your app.