If I want to change to shorter crank arms, do I have to buy a whole new crank set or can I just swap out the arms? On my bike it looks like you could technically remove the arms without removing the chain rings but when I search online for arms they always seem to come as part of a set with the chain rings.
If they are compatible, you can swap the chainrings (ie you don't have to buy arms and rings together). I have shortened arms and Q rings on my bikes - two separate things entirely.
Now as to details, I always get confused and would suggest asking your LBS or the place you order the arms from. Mike Stonich is supposed to be pretty good with customer support.