Put a placeholder clip on the primary storyline the length of the 6-way segment. Edit the video sources (greenscreen) as connected tracks over the placeholder getting the timing and sync right against the music. Apply the key effect to each source and place the appropriate background under each. You will have to turn layers on and off as necessary to see what you are working on. Once you have the keys all set up make each of your six pairs (foreground/background) a compound clip. At this point you should have six compound clips stacked up over each other full screen with the placeholder clip under them. Apply Simon Usdell’s 6-Way Split Screen effect to the placeholder. Here is the link to that effect:
https://f1.creativecow.net/2702/fcpx-multipack-split-screen-effects-debugged
Turn off all the compound clips so only the placeholder is visible. Select the placeholder clip. In the inspector you will see the parameters for the split screen effect. Just apply each compound clip to the image wells in the inspector and you are done.
It’s actually a lot easier than it sounds and Simon’s split-screen effect makes the layout of the 6-way split a snap. I use the placeholder clip as a container to keep things tidy.
NT