Can I attack shoulder armor?

Yes. When defending the shoulder armors have a base defense (pilot defense + any crew/hangar bonus). Shoulder Armors count as pilot points. If a shoulder armor is destroyed the armor is discarded into the owners scrap heap (it does not get a BLOW OFF card like the limbs) and no longer count as pilot points. If two shoulder armors are stacked on top of each other and the active shoulder armor is destroyed, both shoulder armors are discarded and no longer count as pilot points.