How do I avoid double NAT in this situation?

I have an upcoming job where I’ll be installing and configuring a FortiGate to serve as the edge router at a facility currently running an all-Cisco internal network (from a previous contractor). The long-term plan is to replace all Cisco devices with Fortinet gear, but to avoid downtime, the Cisco network will continue operating through the FortiGate for now.
Currently, a Meraki MX400 is acting as the edge router. Since we’re unsure of its configuration and I haven’t worked with one before, we don’t want to remove it yet. Instead, the MX400 will connect to the FortiGate, which will then connect to the ISP. We’ll begin building the new network off the FortiGate while keeping the existing Cisco network running through the MX400.
The MX400 already performs NAT, so I want to avoid double NAT on the FortiGate. Would the correct approach be to create a firewall policy on the FortiGate for the interface handling Cisco traffic and simply disable NAT?