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

Yes. You can sync an external device (slave) to the SQUID (master) by outputting clock output from the USB-B, MIDI OUT1, the MIDI THRU/OUT2, the CLOCK OUT, the DIN SYNC OUT1, and the DIN SYNC IN /OUT2 terminal to the clock input terminal of the external device.