Either @me.com or @mac.com should work, but I would advise signing out in System Preferences>iCloud and signing back in again with the @me.com address and seeing if that works.
On the iPhone you should go to Settings>iCloud and tap 'Delete Account', then set it up again with the @me.com address.
Obviously retain the Yahoo ID for iTunes, iTunes in the Cloud and iTunes Match - these work independently of the iCloud login.
Thank you Roger !! Signing out in System Prefernces and then signing back in with @me.com did work... Mail, Contacts and Calendards now show up in iCloud correctly. Will work with this for awhile and then move on to your recommendation for iPhone. I am being extra careful because of so many bad experiences with MobileMe in past years. Thank you again for your help.