Can Network Theme be used for this?

I am trying to figure out the best approach to achieve our goals. We are creating a multi-site whereby our users will create their own sites and their own social community (mini-hubs). Originally, we were thinking that each user would be given one site -for example, "My Hobbies", with multiple pages for their different hobby categories. But since pages are static and they would not be able to have post pages for each category and we would want a different sidebar to correspond with the different categories - we are realizing that we need to set this up differently, whereby they would get one main site (My Hobbies) and add new sites for each of their hobby categories. Am I correct in this realization?

If so, would the Network Theme work as a good default new site theme for each of the new users and that as they add new sites, their posts from the secondary sites would feed into "their" main Network theme "My Hobbies" site? In our multisite network, could we have 50 users creating their own independent network themed site with secondary sites? We do not want to aggregate across all 50 sites. I hope this is clear.