I used to run a Smoothwall system. From what I can remember it was built to run on PCs using later x86 and above CPUs. No ARM version.
It should be possible to build an ARM version, but would be a lot of work. The sources for the free version of Smoothwall are available, but I seem to remember that it had to be built on an existing Smoothwall system. I tried doing that myself when they dropped support for some of the earlier x86 chips (AMD K6 in my case), as "No one runs hardware that old anymore"...
Perhaps you could look at how Smoothwall works? It's mostly a simple text menu system and web GUI that configures standard software (which is available for the Pi) like Squid (Caching proxy), DNSMasq (DHCP and DNS server), NTP, Apache (for the web GUI) and most important: iptables which can be configured by hand, or by using shorewall.
I just gave up on Smoothwall and bought a cheap router. It does everything that I was using my Smoothwall box for and much less than 1/10 of the power consumption and no noise, which is what I expect you were planning to do with your Pi.