How to deal with signal polarity relative to USB-RS485 converters?
That's just a question of how the pullup/pulldown resistors are configured, which determine the state of the bus when the driver is not active.

Clearly, the ADAM has bias resistors that keep the bus in an idle state, while the FTDI does not seem to have any resistors at all.

It should be fine to connect them to each other. Even if they try to talk at the same time, the drivers are generally current-limited in order to prevent damage.