It will depend a lot on how you have the 'freezer temp' dial set on the fridge.
The way a fridge would normally work is it only cools down the freezer, but a portion of cold air is bypassed into the fridge portion.
The thermostat is in the fridge and determines when to turn the compressor on or off.
so, the two knobs in the fridge control the basic temp balance. One knob is the thermostat, the other knob controls the amount of air that bypasses.
Since you are using it as a fermentor (and assuming a controller), the knob to the thermostat won't have any effect. the other knob (generally the one that says it adjusts the freezer temp) will determine how cold the freezer stays.
for most effecient operation, you'll want to set that to the warmest freezer setting so most of the air goes to the fridge.
Now since it sounds like you want to use the freezer compartment for something, keep in mind that it's temps are likely to be much more erratic.