Can I mix System Sensor SK-protocol devices and Hochiki SD-protocol devices on the same 6820 panel?

No, you cannot mix System Sensor (SK) and Hochiki (SD) devices on the same 6820 panel. While the panel supports both protocols, they cannot be mixed together within the same system (Page 5) [1].

This restriction also applies to SWIFT wireless devices, which are only compatible with the SK protocol (Page 3) [1].