They will not fire in PD mode when set to OL. You can toggle them from OL to Normal power by using the Max Point Defense hotkey.
NOTE: You must cycle the key once, i.e. press it twice to get them to function. This is because Phasers are set to PD by default at mission start up and you disable that by putting them on OL. If you set your Phasers to OL and do nothing else, only your PH-G's will fire in PD mode. Once you cycle the Max Point Defense key, all of your phasers will be connected so you will need to press the hotkey everytime you want PD to fire at all, PH-G's included, unless you want to leave your Phasers off of OL.
CORRECTION: While the above is true, I just tested it and you need to toggle the PD off and on at mission start to get them to work, even if not put on OL.