The general "stacking" (Juniper's Virtual Chassis) and "MCLAG/MC-LAG/MLAG" are two different things. Some providers like Cisco, Juniper, and others might support both depending on models. Below is one of some explanations for the differences you can find online. FSW supports only MCLAG, not stacking.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.