There is a slight limit I think on small amounts of disks.
Each node is master of half the disks in a tray and by default the layout of raid sets is at a node level.
So at the moment each Node sees 3 x FC disks and 5 x SSD disks. So CPG set sizes have to fit into 3 or 5.
There was an update in one of the 3.2.2 releases which added some more flexibility, allowing the set sizes to be across Node Pairs I think but I've only seen that on our SSD based 8xxx boxes so don't know if it applies to FC as well. After we had that update I was able to move our volumes from R1 to R5 (we only had 6 SSDs per tray), before the update I couldn't create a R5 CPG either.
Just had a look at the release notes (3.2.2MU2), I think the feature was "Express layout (for SSD only)" and as I suspected it does state SSD only.
Couldn't see any enhancements to that feature up to MU6 at least so I think you may be limited to R1 on FC unless you add more disks.
EDIT: On better news just checked the release notes for 3.3.1 and it states Express Layout enhanced to support all drives, not just SSD.