Can you please make more examples of what you did, but change the layout up a little. Explain why you would clear in certain situations, and can you organize your css a little. Not that its all over the place, but you lost me on the styling part. You starting creating tons of different tags for different things. One example is you have a h1 and h2 tag, you made a style for h1 h2, does this override the other two? These are things that confussed me also.
Thanks for the feedback. While the styling portion does cover a lot of ground rather quickly, you may want to take a look at the written article series, also by David Powers and also covering the same example. It goes into much more detail about the styling and may clarify some things for you. Here's the link to part 1: http://www.adobe.com/devnet/dreamweaver/articles/first_website_pt1.html
As for your question, if I am understanding it correctly, yes, a style for h1 h2 would apply to all h1 and h2 tags on the page. If there are existing styles on that apply to those elements, this would only potentially override any elements that are the same; meaning if a font face was applied and you specify a font face, it could override it. However, CSS gives precedence to styles that have more specificity, and a tag level style rule is not very specific. Here's a good article covering this topic (and it is arguably a complicated one to get a handle on): http://www.vanseodesign.com/css/css-specificity-inheritance-cascaade/