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For the most important tags I would recommend writing meta descriptions to help increase traffic and leave it for the less important tags.

The number of categories on your blog can grow over time if you write about a diversity of topics.  To justify a category you should have a few existing posts in that category or anticipate generating a few posts within a reasonable amount of time. Extremely active blogs can support, and probably should have, a large diversity of categories simply to organize the content better and gain keyword reach in the search engines. Another factor is the power of your site.  If you have a new site without a lot of power then busting out 100 categories could be like dead weight on your entire site. I have a blog that over the past several years has received 25 to 60 posts per week.  It has over 150 categories and each post goes into at least two categories (one for geographic location and one for topic area).  Most of the blog's search engine traffic flows in through category pages because they rank well for high traffic terms.

There is nothing wrong with having ore then one page ranking for the same keyword, in fact inking from one to the other can add relevance to the linked to page. the only problem is that because search engines like to share the results around,, you may only get one page returned in results, you may need to optimize in a way that the right page is returned.