Squid and MRTG can be easily combined for monitoring.
See this link first :: arnes.si/~matija/utrecht/labwork.html
Here you don't need to add patch as MRTG 2.11.x don't need it.
rest is written there. but make sure squid is compiled with --enable-snmp , I am not sure about rpm versions.
And if any problem , plz ask here