I'm very new to this, so take this with a grain of salt....
I run a fridge/freezer combo and I control externally via the fridge temperature. I have set the internal fridge temperature setting to the lowest possible. My reasoning for this is that I want to make sure that the fan sucking air from the freezer to the fridge is permanently running to get the best possible temperature control. It is likely that it would anyway though due to the elevated fridge temperature. My fridge/freezer is unfortunately intelligent. I've had to do a few mods to it (removed the buzzer off the control board or it would constantly alarm, and control via the FET which runs the compressor) to make it a little dumber.
I hope you have success with your new cave.