Well three of the hats I listed are i2c, some with selectable addresses so I thought stacking those should be fine but I haven't looked close enough to know whether the oPi One's pinout is 100% compatible.
I do have a modified firmata sketch on an Uno communicating with Node-red on the oPi. So far it can do the digital & analog io, and I've added onewire support by reporting the temperature readings via the firmata String support. I would just need to add support for the steppers in a similar way. I also have all the necessary hardware for the Arduino to interface with my project so that's what I'll probably use but I thought I should at least look at the option of doing it without the Arduino.
