planting a shade garden under a hickory tree

Hickories are intermediate in their sensitivity to root damage. A 3-4 inch depth of compost is likely to be OK. Soil is heavier and denser, so damage is more likely with that depth of soil because of restriction of oxygen to the roots. It would be safer to amend or change the soil in only about 1/4 of the tree's root zone at a time, and give the roots time to regenerate before disrupting any more roots.