I think Bruce means that a 700 Series sensor/plug/switch is backwards compatible with older Z-Wave controllers. Based on your experience, it would seem that the Z-Pi 7 is not backwards compatible.
There’s a thread talking about why 700 Series devices aren’t supported at this time.
Note in that thread that there’s an unsupported test version of the binding on GitHub that might work with 700 devices. However, you’re trying to combine both 500 and 700 devices, so I doubt that will work.
Out of curiousity, why do you need a master and a slave? Maybe there’s another way to accomplish what you’re trying to do.
I currently have a Fibaro HC2 (80 nodes) in the main house, and a Fibaro Lite (30 nodes) in the outbuilding, I had to do this, because although distance is only 4 metres between the two buildings, I always had problems. When planning my move to Openhab, I already had a Aeotec Z Stick which I was playing around with, and I saw the new Z-PI 7, and thought I could do the same and use two Z-wave receivers, put the Z-PI 7 in the main house and run the Z Stick (over USB/Ethernet) for the nodes in the outbuilding. I don’t have any 700 devices at the moment, just thought I’d get the latest so I’m covered for any new devices I might want to add in the future.