GUI should be reachable but keep in mind it's behind the FortiLink interface and so you would need a policy to access it. This has to be done via CLI. That said, you probably don't need to access the switch GUI once its managed by the FortiGate. All VLAN and port config happens on the FortiGate now.
Regarding your DHCP servers. The FortiLink interface has a DHCP server and this is reserved for FortiSwitches only.
You create new FortiLink VLANs with your own addressing and VLAN ID assignment and DHCP server configs to service your various internal networks.
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