Hi,
iChat has never been "portable between OS versions.
iChat (nominally 1) does not work outside of Jaguar.
iChat 2 beta work in Jaguar.
iChat 2 and it's numbered updates only work in Tiger (OS X 10.3)
And so on until iChat 6 in Lion.
Messages continues the numbering as 7 in Mountain Lion.
It has one update at OS X 10.8.2 to version 7.0.1 At this point the Serial Number of the Mac must be readable on the Logic Board (repairs to the logic/Mother board that don't have it put back need to revisit the Store or repair place and have it put back).
AS with iChat versions the Messages 7.0.1 will not work with being at OS X 10.8.2.
I.e. not only will it not work in Lion (OS X 10.7.x) but will not work in pre OS X 10.8.2
As you have found out this means iChat 5.x.x from OS X 10.6.x will not work in the newer OS of El Capitan.
By OS X 10.11 and Messages 9 (there were two versions 8s between vers 7.0.1 and Messages 9 there is very poor numbering about updates) we have lost:-
Formatting Menu and it's Messages Preferences Pane that allowed different Balloon colours and Font Colours and Styles
see http://www.ralphjohns.co.uk/versions/ichatvers4/ichatvers4pics/Prefs/slides/iCha t4MessagesPrefs.html
And http://www.ralphjohns.co.uk/versions/ichatvers4/ichatvers4pics/menus/slides/iCha t4FormatMenu.html
The Video and Audio Only chats form AIM and Jabber/Google Buddy list that worked Mac to Mac has gone.
What formatting there is in now in the General Pane of the Preferences

Sounds (what is left, again) is also in this section.
These are the Ringtone and Alert sounds that have come from iOS
The section below the red line only appears if you have AIM, Jabber (including Google account) or enable the Bonjour account.
Start with Enabling the Bonjour Account first.
This add "Buddies" to the Window format and will then display the Buddies List.
Technically at this point it is a combine list of all the Buddy List accounts.
If you have set up multiple account the list can get busy (Menu and Buddies List together)

You will notice I have untick the "Show all accounts in one list" (It may say "gather" on older versions)
The Window Menu will now list all accounts (AIM, Jabber etc) in separate Buddy Lists.

Lots of Accounts in Messages and the Main chat window can take up lots of Screen Space though

more to follow.

8:33 pm Wednesday; July 26, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 10.x and an iPad (2)