I am not an expert on these protocols. Atleast not yet. Maybe I will become one soon, but the fact is why cant one use GAIM to do Gtalk, Yahoo, Jabber when on the same network people are having a gala time using the same services on Windoze? Why??
I work behind a very restrictive firewall too, and we (people in my org) realized that the communication in all the clients is happening over http. The clients "tunnel" their protocols over http and hence it works.
If you use gaim, you may want to go to advanced settings and try giving port numbers as 20, 23 and 80 to see if yahoo, gtalk and MSN start working.
I have not tried it, hence can't comment. But this has been tried in our organization.
- Navneet