HI,
iChat, at iChat 6 in Lion, can join the AIM service, the Jabber/XMPP service, the Yahoo service and has the Bonjour (Mac to Mac on LAN) service.
Each services is separate from the other.
This means if you have a Yahoo Account set up in iChat then your Buddy will also have to have Yahoo account. (Same for Jabber and for AIM)
In addition some Apple issued IDs (namely @icloud.com, @me.com and @mac.com) can be used as AIM Names in later version of the OS X (there was a Change on the 30th June regarding using @mac.com and @me.com in versions less than early OS X 10.7 (Lion) )
Jabber IDs can include GoogleMail IDs and Facebook Chat IDs
None of these can communicate with the iMessages service.
The Messages app (in Mountain Lion and Mavericks) also has the iMessages account.
This uses any Apple ID (I.e. any ID from Apple or any email you have registered as an Apple ID)
This means it could also be your Google ID or your Yahoo ID which in turn means Messages can be set up to use an ID twice depending on the services it can be used with.
As Messages does all the other accounts that iChat did you will have to use one of those to connect to your iChat Buddy (Not iMessages Contact).
8:50 pm Wednesday; July 2, 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