How to have the 1st to register on a site be an admin and those that register after be contributors

I have a multi network and multisite setup. I want the first person that registers on a new site to be automatically created with the role of admin. Then any users that register to that site later would only be able to register as contributors or authors.

I'd like this to happen automatically. I am the super admin for the global network and each state network. I should point out each state has a network and there are multisites within each state network that are for schools.

So the first person to register at would be made as admin for skylinehighschool but would not have super admin permissions for all of idaho. Anyone to register on Skyline's site would only be able to register as a contributor or author to only the Skyline site.

Maybe I can't have this set automatically, but if the user registering has a drop down menu that lets them pick which user role they are registering as, could I have the "admin" option disappear if someone is already registered as the admin?

Clear as mud?

Thanks all