Hmm...
I think pit fiends are too iconic as devils for them to be "demon transplants." What I'd rather see is one of the following:
A) Pit fiends are an exception to the "devils tend to appear human" rule, being such ancient examples of diabolical evil--but lacking the power of the Dukes, to maintain their appearance--that they've taken on a more monstrous form.
Or, my preferred option...
B) Pit fiends have two forms: A very human one, with only a few diabolic traits, and the standard pit fiend form, which erupts when they grow angry or violent.