Administration username should contain letters, underscore & digits and it should start with letter.
I think you would have to take the approach of creating a User within the application and assign the user role of Administrator to that user, this user can have Username as email address as this will give you more control over the Username attribute. Let me know if you need any more information on this.
The the username and password in the Administrator tab should necessarily be used by a SuperAdmin like a backend admin or developer. Hope this helps.