Yes you can use the Guardian, but it will cost you $30 - $35 for the SME interface. You can also use Squidguard rpms for free (
http://www.tech-geeks.org/servers/sme/squidGuard/). Dan's will allow a more refined control.
But if all you are doing is a content filter/firewall then look at IPCOP.org. Dan's interface is free (get addzon). IPCOP has a true firewall interface with builtin intrusion detection.
If I were doing more then what you are asking I would use ESmith. Otherwise IPCOP will do what you need and is as easy as SME.
I support 48 school districts in Illinois. Plus there are other netspecs in the state using it.