WordPress vs WordPress MU
Basic set-up in WordPress, customize an author to have his/her own templates for Pages and/or Posts:
1. For Pages - create a Page template for each author, named with author name, which each author will choose from dropdown list at Write
2. For Posts - easiest and fastest way would be to create a category for each author and set up category templates customized accordingly.
NOTE; Of course there are other ways to do it, code intensive and a bit more complicated.
For 15 users, I recommend WPMU, platform used in WordPress.com. You have a main site which can aggregate all the posts, tags, categories from the sub-blogs/websites you can set up for each author using just one theme set up with the variables you want. This set-up gives the bloggers some measure of privacy and independence from other bloggers and gives an easy way to customize site/pages/posts per blogger. There's so much more you can do with WPMU, it all depends on your imagination :-)
Requirement No. 1: WPMU Readme.txt