Multiple sites, each is either one page (with multiple editable content) or has multiple pages I tried researching into this on google for a while, but I'm stuck.

Basically what I'm trying to do is this: I have a "big" main website.  Then, on the same drupal installation I want to have a "mini" site that will have a completely different theme, and only one page (but the page will have multiple editable content).  I also will have another site that will have a completely different theme, completely different navigation, and multiple pages.  Final piece that has me puzzled is that this will be on a subfolder basis.  By that I mean / will have the main site, /sitea will have the single page site, and /siteb will have the multipage site.

Any suggestions on how to handle this?
If you don't need multiple domains (or sub-domains for that matter) you could probably get away with using a custom module to simply switch themes under a given criteria (URI path, user role, etc).

How do I change a theme based on the URL?