You can use a single users table and do single sign in... but it might be a little tricky to update the login page. I think the easiest way to do it would be to eliminate all of the login pages but one, then update that remaining page to set all of the session variables that would have been set if you logged into all of the admin pages separately. That would accomplish the single sign in for admin.
Then probably using the PowerMessenger users table would be the most appropriate because I think it has a few extra columns you need for that application. You could remove the registration, insert, and update pages for the other users tables and then make sure you don't refer to them in other pages.