Questions about users, categories and adsense.

Hi,
I’ve thought of starting a blog to be a review of certain consumer goods. I’ve got a few questions to see if what I am thinking is possible:

-I know you can use categories for blog entries. Is there a way of using subcategories. I mean if we were writing reviews on cars, could I have a category of Nissan and then several subcategories under Nissan for different models of Nissan cars? What about a subcategory under that subcategory. Maybe for different years of that model Nissan car?

-I want to be able to invite others to write reviews by giviing them the opportunity to make adsense money. Is there a way that I could put adsense code on the main parts of the site… let’s say at the top and bottom or the sides and that contain may adsense publisher ID… and each blog entry by users contain ads that contain their adsense publisher ID?