The problem with the soil will make planting difficult as these trees take out all the goodness Can you make raised beds? This would increase your options. Ferns are a good start, especially the few that are happy in dry ground - Drypoteris probably the best and there are lots of different ones. Hostas and heucheras are more good foliage plants for shade. Epimediums. Monardas might do there, worth a try. And try a variegated Euonymus fortunii or two - silver or gold variegations and evergreen. There are even a few perennial geraniums that would be worth a try.

I live on Long Island and just made a shade garden myself in front of house. Rhododendrons, lily of the Valley, Azaleas (check out the Encore Series), Sedums (Autumn Joy), all kinds of Hostas. Check out the garden center at Home Depot or Lowes. Impatiens do well.