You can assign different skins to pages by enabling it in Site Settings. That way different pages can have different master pages and pagemenu can be included or not included. If the skins are mostly similar in CSS you can organize the CSS in a central location and use it for all of the skins you assign to pages to make it eaier to maintain the CSS.
For info about orginizing the CSS into a common location see Understanding the style.config file.
Hope that helps,
Joe