You might experiment as follows...
1. Add image/sync video to a 5x5 split, fit region
2. Click the + at the right of the Sequence 1 tab to create a new blank sequence
3. Click the Sequences tab in the bin and drag sequence 1 onto the timeline
This is now a composite - audio won't be seen, but it's there
4. Use SCALE effect to position and scale the composite similar to what is shown below (juggle the figures for best display/aspect)
5. Overlay the 6th image/video, split it as position U in a 5x5, fit region, sync it
This should result in something akin to the final image below
From there export Seq 2 and proceed as Nat instructed in his 6x3 tutorial
He will likely come up with a better approach



It's doubtful that grid options will be expanded, but file a suggestion by clicking the chevron at the top-right of the VP window.