Connecting one switch to the FortiGate (and adding other switches off it) is the recommended configuration from Fortinet.
You should use a Fortilink LAG group for the connection to the first switch to avoid single points of failure- but obviously with only one FortiGate there is always some risk.
If you connect multiple switches to the FortiGate (and the 60F is an entry level model) you rely on the FortiGate to perform switching between the switches. This is likely to cause performance bottlenecks- even if you can configure a hardware switch.
However, if you do want to change the FortiLink interface to a hardware switch you need to ensure all config is removed from the FortiLink interface. IP addressing, DHCP server, NTP server, firewall policies must all be removed before you can make the changes.
If you look in the interface cofig section of the GUI it will show the number of “references” against the interface. That needs to be 0 before you can make changes.
Hope that helps a little.
Andy Bailey, Christchurch, New Zealand