Marc,
Could the SG200 be not reading correctly? Maybe the software is corrupted and needs to be updated?
I was able to buy the BT dongle separately, after they came out, for my SG200. I had installed the 614 a few years earlier.
I installed the app on my phone and have done a few updates to the SG200 software over the past few years.
If I remember correctly, there have been seperate updates for the shunt, gauge and I think a third device. Maybe the dongle.
May be worth finding the dongle and updating the SG200 before putting out for the 618. Unless of course, that's what you really want for the new features it brings. I do.