It would depend on the interface and how you have it set up. But by default the monitor knob on any given interface will always control outputs 1+2. This are the outputs your monitors would be connected to.
Sorry, i missed the part about 5.1. Again, it would depend on the interface and how it is set up. I have a Focusrite 18i20 and I can do this. There are 3 modes on mine for each output. Depending on the mode of a given output, for my set up, will determine if the monitor know will control that outputs level or not. Which unit do you have?