Can I connect devices (like a Konnected Pro) to my Hubitat C8 via Ethernet or is the Ethernet port only for connecting the Hubitat to the router?
The Ethernet port on the Hubitat Hub is only for connecting the Hubitat to the router. It cannot be used to connect devices.
You would connect both your konnected pro and your hubitat to the same switch. (Or separate switches on the same network).