Honestly, why not do fortiswitch with your FortiGates? Using the fortilink feature fortiswitchs can be managed from the FortiGate

In addition if you get the fortigate cloud license you are then able to use the FortiCare portal to manage the device and can still login locally to make changes. Do not get the fortimanager cloud that is a seperate offering you do not want in this case as local changes will mess with things