I imported the final 5.1 mix delivered to me from the post house, the come into fcpx 10.3.4 as individual mono files: L,R,C,Ls,Rs,Lfe - I believe I should leave the pan mode set to none for each of those files, correct? I f I set it to basic surround I think I will be messing up the surround balance already set my the mixer and baked into these stems. Is that correct?
If you burn a bluray disc out of FCPX, then you need to use the surround audio configuration on the file. Otherwise the channel layout will most likely be incorrect. On delivering to your broadcaster or distributor, you need to use the Multichannel role output method. As long as the channels line up how they want then it's fine. You can check the resulting file by dragging it back into FCPX and seeing which waveforms show up on which channels. And soloing or muting the channels so you can hear the individual tracks.