Sorry for the late reply. I never tried OneShot in the end, just standard PWM, and had an ESC fire due to not understanding one of the parameters. I'm decided to move over to using ardupilot so that I can run DShot, so won't be trying Oneshot on PX4.
However, in terms of wiring, I think you need to power the ESCs from the PM07 pads in the same way you would for the standard setup (from the M1-8 B+ and GND pads), but for best performance, connect the signal wires via the FMU rail, and disable the I/O PWM outputs. Then set PWM_RATE to 0.