I am a little confused by the discussion. This seemed to be talking about making due with some limitations on the cable modem firewall provided for home services using multiple VLANs on an ex switch. But some of the comments suggest you are working on a small business single site solution as a professional install.
From an architecture point of view in a small business site, I would work with the provider to place their device into bridge mode. In cable and DSL modems this connects you to the upstream provider network and gives you just an ethernet port hand off for the business network.
This hand off should go to a business grade firewall that will perform the internet NAT and necessary firewall protection for the site.
For sites with multiple VLANs, you have a choice as to where the default gateway for those VLANs will live. Generally speaking the simpliest is to have them on the firewall. The reason to have them put directly onto the switch as a routed VLAN interface would be because the local traffic is of such bandwidth that the firewall would not have the capacity for the traffic.