I'm going to answer my own question and my own Situation.
What did I do: -> Added column "user_admin" to my User Model -> Since I'm using cancan I can manage my admin users from Ability, more information is found here: https://github.com/sferik/rails_admin/wiki/Cancan
When a user is an admin_user he can manage everything but when I login with my SuperAdmin I can manage all + User model