I have a couple rigs with these boards.  All with 5 GPUs.  All the rigs use risers on all cards.  Three X16 unpowered risers and obviously X1 risers in the X1 slots.

Every board seems to be different in the combo it likes.  One you have to short the presence pins on the second X1 slot one you dont.  One you need powered risers on the X1 slots one only needs one X1 slot powered.


Easiest way is to just have all the parts ready and just try it out until you get it running.