We break up our sitemap files into several different site maps, and then use a sitemap index file to make sure Google finds them all.

At the bottom of this post they talk about using an index file to combine multiple sitemaps, and they also specifically say it is fine to have one time sensitive site map (ie: front page items) and several other less time sensitive ones (categories in your case).

http://googlewebmastercentral.blogspot.com/2006/10/multiple-sitemaps-in-same-directory.html