Does anyone know whether or not the spools for these models are interchangeable?  I called Shimano and they said I could interchange the spools so long as I also interchanged the drag knob...sounded a bit strange to me, hence my question here.

It's not strange. The drag knob is made to fit into the top of the particular spool it is being used with. One model's spool and drag knob may be slightly different sized than the other so it wouldn't fit back together properly unless you switch both parts out together.