installing several wordpress websites

Hi
Am new to wordpress.

I want to start an integrated classified marketplace website for over a 100 universities.

Alongside this main classified website, I want to add other wordpress websites( which can be accessed from the classified menu lists) namely forum, social networking, chatting, dating, business directory, ecommerce, educational blog and elearning

A user should be able to login from any of the above websites

Also, biz directory, ecommerce, education blog and the elearning websites will be such that students can generate individual sub websites

Am not sure how all the above websites should be configured, installed or maintained-is it wise or otherwise to have the main classified site installed in the root directory, the additional websites in the subdirectory and the student generated subsites in the subdomain?