Are you looking to use the Fortigate as the edge of one spoke, the edge of a DC, or as a hub?
- spoke - create a site to site VPN to the MX in the main site, should be okay as long as you don't need access to/from the other sites etc. due to routing issues.
- edge of a DC - if there is an MX as a concentrator there and it is just an internet edge then this should be fine.
- hub - just don't even think about it, you lose all the benefits of autoVPN/SD-WAN
So, unless you are following 2, or can accept the limitations of 1, it isn't the best idea