I have a cabinet with (2) stacked switches (24 ports/each) and (2) Firewalls (Fortigate) running in HA-A/A. To conserve ports on the Firewall would you recommend that we VLAN the WAN1. Our connection to the DCcarrier is limited to (100Mbit)

->Feed 1 -> SwitchStack1 PORT1 -> Firewall WAN1 ->
->Feed 2 -> SwitchStack2 PORT1 -> Firewall WAN1 ->

Firewall WAN1 VLAN (IP's remapped)
.VLAN 10 ->
.VLAN 20 ->
.VLAN 30 ->

The same stack-switch will have internal traffic on separate logical VLANS also connected to the Fortigate firewall.

I'm just looking for suggestions or best practice in a VPS hosted environment.