a compressor works as if you were adjusting the volume knob, by turning it down every time something loud came through. Normally its the track itself. However, you can also make the adjustments based on another track. Thats called the sidechain input. Try reading the logic manual on sidechain option for the compressor.
Not sure about compression/expansion.
I am not sure I want to compress or expand....
Just adjust the volume of the master track using the fireworks control track...