WordPress MU

I have searched this forum over and over for information on how to do this. So any help would be appreciated.

I would like my users to have the ability to select between multiple domains when registering(example theirname.aaa.com or theirname.bbb.com or theirname.ccc.com and so on). I would like to use one primary domain with about 19 other domains to choose between, but all data, such as recent posts is displayed on the primary domain.

In case it's a similar answer, what about theirname.blogs.domain.tld and theirname.blogs.domain2.tld ?

define a site per domain in wp_site and associate blogs with their specific domain.

If I where to do multiple installs is their a way to fetch recent posts from the other domains and display them on the primary domain?