first - is a FortiSwitch set up behind the FortiGate's DMZ interface, or do you have a third-party switch set up there?
-> if third-party, then you need to set up VLANs and tagging there. On the FortiGate, simply create multiple VLAN interfaces on top of the DMZ interface, and then do your policies between the VLAN interfaces (FortiGate will associate traffic with specific VLAN interfaces based on VLAN tag; untagged traffic will be considered to belong to the physical DMZ interface)
-> if it's a FortiSwitch - what firmware is your FortiGate, and what firmware is your FortiSwitch? The guide is written for 6.0, and a number of CLI commands have changed since then
The access VLAN configuration you pointed to is primarily intended for connecting new devices on a switch port - a new device connects, it goes into the access VLAN and can ONLY speak to FortiGate, FortiGate determines what VLAN is appropriate, and shifts the device into that (other) VLAN (where it could speak to devices in the same VLAN without going through FortiGate).
I'm not entirely sure if that is what you're looking for? From your updates above, it sounded a lot as if you want to have the servers in fixed, separate VLANs and handle all access between VLANs through FortiGate.
As long as the servers ARE actually in different VLANs, all the traffic must go through FortiGate and policies anyway; FortiSwitch (or any switch with VLAN capabilities) should not allow cross-vlan traffic, and the access-vlan configuration should not be necessary
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++