Dear Amit,
In BITS Pilani we have a similar problem. The way we connect to
gtalk/yahoo from Linux/OS X via our proxy is by connecting to the actual ip
address of talk.google.com (obtained via nslookup) (same thing for yahoo),
and port 443 (secure http) and forcing Old SSL. You can also try the same.
Note: use ping talkx.l.google.com to get the ip address
Regards,
Karthik Ramgopal
BITS Pilani