Those are some pretty good questions I never really asked myself. The Tobi one was kind of subconscious thing I'd ask myself.

As to Zetsu and Kisame - there are actual demons in the series. So they basically could count as demons themselves or just as a lesser kind of demonic being. Though, doesn't explain how Orochimaru looks like a snake...

Oh, and the Jounin thing. There aren't any exams, they just get picked by the Kages due to their skill and also by recommendations.

Sorry if you already knew this, though xD I couldn't help myself.