Mulch is the easiest. Anything that you plant will have to compete with the roots of the hemlock, also you will have to weed in between the pachysandra for quite some time. until it is established, and then there still may be some weeds. Also If you plant pachysandra, you will have to mulch around the plants until the ground cover is established.

I inherited an island planting of hemlocks. I cut out the English ivy (that was climbing into the Hemlock canopy), and planted pachysandra, ferns, and Japanese Forest grass beneath (high shade tolerance unlike most grasses). it did take a few years until the pachysandra spread (ferns were fastest).