Single FortiGate unit managing a stack of several FortiSwitch units - Third Party Switch

I'm about to set up two FortiSwitch's up using the "Single FortiGate unit managing a stack of several FortiSwitch units" topology. I will start with 2 FortiSwitch's but may add more in the stack.

As I read the documentation at: https://docs.fortinet.com...work-topologies#Tier-1 there is a note that says: External devices shown in the following topology must be compliant endpoints, such as computers. They cannot be third-party switches or appliances.

Can a third party switch which supports VLAN's be connected to one of the ports of one of the FortiSwitch's and can the VLAN extend through the third party switch?