Could you be more specific regarding each drive? Capacity, etc?
Only 1 drive is required in the array, the mechanical drive would be a good choice for that. All others could be Unassigned Devices. Or some combination of Unassigned and cache pool depending on specifics.
The main thing you lose by not having disks in the array is the ability to span folders, but sound like that isn't important since you want to work directly with the different disks anyway.