Setting up multisite creation.

Hello.

I have been working on a site for awhile now. The goal is to have a site like wordpress.com or edublogs.org and have used a lot of your plugin to come close, but I cannot get what I want to work correctly. What I am looking to do is allow people to sign up and start payment, that will create a new site with there sub domain and this seems to work with Pro Sites, however here are the features I cant get to work.

- There should be one theme that is automatically active and cannot be changed (all Logos and settings should not be accessed or changed by the user, so no access to /wp-admin)
- all content and plugins should be the same and active on every new site.
- No sites should be able to be accessed by anyone unless they know the subdomain and are signed in.
- limit the amount of users that can be added before the payment package needs to be upgraded.

Is this something support can help me with or a way i can get some custom work done?
Thank you so much!
Mike