The early Newtons did not need a special dongle, just a regular mini din9 or whatever serial cable(like ones the back of your Macs)

With my OMP and old 68k machines I was connecting the Newton to the "modem" port because I was already using the "printer" and ethernet ports for networking. I cannot recall having any issues aside from a bad cable I encountered once.

To be quite honest I personally would just use the 6500 and not bother with USB adapters and software in OS X that, IIRC, does not truly fully support communication with NOS 1.x.