Very simple, and elegant. forget about the packages you're talking about and if you merely follow the simple examples in the two links below you're off and flying.
Also, I wouldn't bother with doing this on smoothwall either. I would let the packets pass through to another Linux box behind the firewall to alleviate the load. Let the firewall do the firewalling and simply allow from the various user IPs passing it through (redirecting the packets) to the machine behind the firewall to handle the tunnels themselves - but that's just me.
Even reading these two HOW-TOs slowly, you can have your users tunneling in within 15 minutes quite easily
Otherwise, you may want to get ASTARO Security Linux (I'm not a smoothwall fan for anything but the home user - I don't like their inflexible 'zone' concepts), and then you can point and click - but it will take you much longer to set up the VPNs, and you'll prolly lock yourself out once or twice from the remote firewall server till you get the hang of it, coz you gotta do both sides of the equation - don't forget that so leave your own IP completely Permissioned to traverse the WAN and then test on another box to verify.
Seriously, GRE tunnels are easy, and supported by CISCO too.
I hope that helps