if my memmory serves me correctly ... I believe that you have no choice over the filesystem used during the installation, the whole emphasis behind the smoothwall project has been providing a fully functional, easy to install, firewall so I suppose confusing slightly less 'clued' users by offering options such as this wouldnt be very helpful
and btw, you might want to check out
http://www.ipcop.org which is (in my opinion) much better than smoothwall (even tho it was forked from the original smoothwall code), and its totally free
so you'll get the same support as everyone else