I think they need to be in the same scale. I have a DDS and I plan to couple her with a smart doll boy. She's supposed to be shorter but since they are the same scale I think I can be happy with that result. I tend to dislike SD/MSD couples because they just don't look like they're in the same scale to me. I've only seen one couple like that that I thought worked ok and the photographer used forced perspective on a lot of the photos to minimize the scale difference.