How to implement InterVlan Routing with a firewall and a router I don't have rights to.?

I am creating Vlan's and I need to set up intervlan routing. My equip. consists of Cisco 3500XL (L2) switches, Two Cisco 2600 routers and Netscreen firewall. One of the routers (our Internet gateway) is owned by AT&T and they will not allow me to do any configs on it. I have a 2nd sitting on my desk waiting for me. An added problem is we have a DMZ and multiple remote offices that need access. Right now, local network and DMZ connect to seperate switchs, the switches connect to the firewall, the firewall connects to AT&T router which runs to the demarc. My thought was to configure the subinterfaces for my vlans on my 2nd router and place the router between the firewall and the AT&T router. My new topology would be switches > Firewall > Router for vlans > Gateway router > Demarc. What issues am I creating by doing this... or is this the correct way to do it?