I had the same issue, When I did use Displacement for the feet I isolated only the soles/sides of the foot and kept the toe area at 50grey so they woul dnot be effected by the displacement. The main problem I ran into is that I couldn't get the micro detail I was looking for, even if I cranked up the subdivision.
So what I've been doing when I need more detail is to enhance areas of the bump map I want to stand out more so I don't end up making the whole foot look pitted.

I am still working on good displacement with this, haven't completely solved it. Not sure if Carrara would be able to use, say very fine detail maps from Zbrush on a poser model.