The SMC appliance that Comcast gives you with the business setups is a combo modem/router. It already runs NAT, its default LAN IP is 10.1.10.1, web admin username:cusadmin or custadmin, password highspeed
You can leave it by default, uplink a switch to it, and setup your network behind it just like any old home grade Linksys router...it runs DHCP by default, clients will pick up 10.1.10.100 and on up IP addresses. The NAT/firewall is decent...just like any other basic NAT router.
The public IP address that it picks up is not the static IPs that Comcast gives you with your account, Comcast will give you your static IP(s) on a sheet of paper when they do the install. What I do is use my own routers for my clients...and you enter that static IP info on the WAN interface of your own router..and uplink the WAN interface of your own router into one of the LAN ports on the back of the SMC. Before that, I log into the SMC and turn off the firewall feature..if I'm running my own services behind it. The SMC will pass the public IP right along, with this setup there's no double NAT'ing, so even though it looks like a router plugged into another router..it's not.
If your case, if you want to leave the SMC as your gateway, uplink a switch to it for your network. For your wireless router...change its LAN IP to 10.1.10.2 or 10.1.10.245 or 10.2.10.253 (whichever you like). Disable DHCP on your router, and uplink it to one of the LAN ports of the SMC..using a LAN port of your router..not your routers WAN/Internet port. This will make it an access point, not a 2nd router so you avoid double NAT'ing.
For your *nix server, you can either do port forwarding, or DMZ it, or assign it one of your statics and plug it into the SMC. I cannot remember if the SMC had a true DMZ mode for a static IP, I've not used that feature. But you'd want to consider isolating your *nix server from the rest of your LAN properly, for security reasons.