Hi,
iChat 1 through 2 uses AIM only (or AIM valid names)
iChat 3 adds Jabber (which means Google valid Mail IDs and Facebook Chat IDs as valid alternatives)
iChat 4 only adds a separate Google option due to a server name issue at Google
iChat 6 adds Yahoo account (no other names that Yahoo allow themselves only Yahoo ones)
AIM ones can include Apple Issued IDs such as @mac.com and iCloud Issued @me.com and @icloud.com
The iCloud ones will not work in iChat 5 or Earlier dues to the security checks iChat 6 and the Messages versions make.
If you linked an external email to an @mac.com account either during the .Mac service or during MobileMe whilst Apple still had the "iChat Name" registration page then you might have issues if you removed that email and replaced it with another on the Apple ID site.
What is more likely is that you have created a new password that exceeds 16 characters that the AIM servers use.
9:45 pm Friday; April 18, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad