An Autarch could give orders to Incubi ... If he's grown tired of having a head. But yes, the languages of the two are pretty similar from the fluff I've read. There is one key point I forgot earlier too. The Dark Eldar don't feed on the souls of Eldar the way they do every other life-form. They may be usurpers and traitors, given the oppurtunity, to other Dark Eldar. But they typically just leave Eldar alone.
Evil is like shadow. It hides in the tiny cracks, biding its time until the Light fades.