I've been told for the longest time that if my shoulders move during singing AT ALL I'm doing something wrong. The problem is, I feel like I can't get a full breath without my shoulders going up a bit (I'm not consciously moving them, they're just raising with the expansion of my ribcage.) When I try to keep it exclusively in my lower abdomen it feels tight and shallow. What am I missing? Is it natural for the shoulders to move at least a little as long as I'm not using my shoulder muscles to push them up?
Welcome. Our lungs serve several purposes: they keep us alive; they pump air past the vocal cords so we can sing, and they help support our body when things get strenuous (think about a bicycle tyre getting more rigid as it fills with air). Because of Pascal's principle, your lungs will want to expand in all directions as they fill with air. Your ribs, sternum, clavicles, scapulae and their associated muscles and ligaments are bound to move to some degree when you take a big breath.
In essence, a singer wants to pump lots of air, and shape resonance with the mouth and tongue, leaving the vocal cords to do their thing uninterrupted. If you start thinking about your throat you're more likely to start activating those big, industrial-strength muscles in there that we use to seal the airway off when we're doing heavy duty things like lifting weights, childbirth and going to the toilet. It will strangle your sound.
You will know if you are lifting your shoulders. It's different to your shoulders being being moved by a chain reaction from your expanding lungs. Try it. Lift your shoulders a few times. You will feel it on either side of your neck. Now compare it to what happens when you take a big breath of air.
So, don't worry if you detect a small movement in your shoulders. The important thing is to concentrate on your stomach region (yes, stomach, because your diaphragm drops down to allow your lungs to fill) and your mouth, ignoring everything in between, especially, especially your throat. Your shoulders are extraneous to singing.