The router machine is a general router machine which provided by ISP, so it can't to be configed with firewall function ( only routed / routing function )...
If I were you, I'd head over to http://www.shorewall.net/ and check out that site. They have lots of good information on setting up a Linux firewall with diagrams that may shed some light on your given situation.
I would install the shorewall package on FC6 and configure it using the site as your guide. (You should be able to simply do "yum install shorewall") Then if I had additional questions on configuration I'd join the shorewall mailing list and ask on that list as they are friendly and eager to help.
Regards, Ed