The Recipe Calculator would beg to differ.
There's 32-34 PPG in dark roasted malts. It's less than the 38 or so for base malts, but starches present in the malt didn't go anywhere when they were heated and they definitely add to the fermentable sugars.
There's not much difference in Carafa and Chocolate roasts. If the number is similar, you'll get very similar results. I tend to keep regional malts together in recipes, using Carafa malts in German beers and Chocolate, etc in English beers but it's just a preference and won't make much impact on the final product if you mix rather than match.