I know you can buy only one tray if required but I have never contemplated that setup.I use the 7+1 RAID mainly because of the cost of the drives. 3+1 versus 7+1 means a couple of thousand dollars wasted with our contract.
Ten trays leaves a lot of wasted space in one of those systems. Sooner or later you will be spending big money to get the best out of it.
HTH!!
Stephen