Its not much different than modeling separate compressor on drivers running at the same speed. You could have some form of gears to alter the speed at one or more of the compressors or unloaders to reduce (or increase) volume per stoke for a reciprocating compressor.
You will need to enter the compessor "map" for every compressor stage. The map is basically columns of head vs acfm. One set of these for each speed. Can you add a column for the efficiency as well.