I have experimented several times with the addition of chiles to chocolate recipes. In terms of which kinds to add, Ancho is my personal favorite and I think it pairs very well with chocolate. I like the addition of Pasilla and Chipotle, but not as much because I prefer a little less smoky flavor added to the chocolate. But I think this is all a matter of preference. As far as Cayenne goes, I think the only noticeable change will be in terms of spice level, and not flavor.
When I have added Ancho to a chocolate recipe (I've done it with creme brulee, brownies, cake and hot chocolate), I find that cinnamon is also a great addition. As far as the amount to use, that really depends upon how assertive you want the chile flavor to be and this requires a little experimentation. Assuming you're baking two layers, I would suggest anywhere from 1-3 tablespoons of Ancho, a little less than that if using Pasilla and even a little bit less if using Chipotle. Depending upon how much heat you desire, I suggest 1/8 to 1/4 teaspoon of Cayenne (a little less if you use Pasilla or Chipotle).