Can I sync my SQUID (slave) to an external device (master) such as Windows PC/Mac or a MIDI device?

Yes. Please connect the clock output from the external device to the USB-B, the MIDI IN, the CLOCK IN, and the DIN SYNC IN /OUT2 terminal of the SQUID. You can select an external MIDI device (as a master) in [Sync Common] setting category < [Sync Source] of the GLOBAL menu.
For details, please refer to the [Sync Common] setting in the "GLOBAL settings" in the Operating Instructions.
You can download the Operating Instructions from the Pioneer DJ support page https://www.pioneerdj.com/en/support/documents/production/toraiz-squid/.