I have to agree 100% with Denny here (can we rename him Beer Yoda?). For a stout, you want roast character. For a porter, you will want more of a chocolate flavor. For a schwarzbier, you want more of the flavor you will get from using carafa (mildly chocolate, if at all).
And from personal experience, I brew a schwarzbier as one of Yellowhammer's flagships. And when I brewed a porter with crafa because I didn't make cholcolate malt, everyone complained that it tasted too much like my schwarzbier!
Doesn't mean you can't do it, but just know what to expect when you do! It will still be a delicious beer if you sub carafa, but it may not be "stout like" if it isn't roasty enough. For me, roasted barley is what makes a stout. Not carafa or chocolate.)
All that said, the secret for my tastes to a stout is a healthy portion of dark munich.