Login redirect for users in sub-sites


in my dev MU environment (where localhost is main site and localhost/userX is the userX' sub-site) I'm wondering if I can do the following:

- force all users to login from the the main site only
- redirect automatically users to their sub-site
- if not logged-in, users should always be redirected to the Main site home page when trying to type localhost/userX site

I already checked "Peter's Login Redirect" plugin but did not look to be the solution.

Can you please help?

Many thanks!