Correct.. This is not recommended. You can ignore the above idea.
Primary MX and secondary MX need to be configured using warmspare VIP.
You may need to have /29 subnet - as we will 6 usable ip address. / 30 subnet wont work.
Below switch need to be separate not in stack to forward the VRRP packets.
If you have the Cisco FTD in active / passive scenario, you need to add two more connections.
Cisco FTD Primary to Secondary MX
Cisco FTD Secondary to Primary MX.