If your using fortigate firewall as core or perimeter devices and cisco sg 300 as distribution switch in your setup then you can accomplish this task by creating sub interface on fortigate firewall
Connect sg300 switch to fortinet firewall with trunk link allowing all vlan
Have a default route in sg300 switch pointing toward fortigate firewall LAN interface (inside interface)
And configuration public ip of isp on outside interfàce of fortigate and configure default route in fortinet firewall pointing towards isp gateway
And now in sg 300 switch create all vlan used in fortigate subinterface
And connectiing servers and desktop and access switch according as per your requirement .