A lot of times, I mix scale and traditional items by "breaking up the silhouette" of the train. What I mean by that is not placing similar car types of different sizes together (i.e. not putting scale boxcars next to traditional boxcars). I usually put flatcars or gondolas in between, and when running, you hardly notice the difference in scale due to the big difference in car types. I've run traditional cabooses at the end of otherwise scale trains, and they didn't look bad (had a flatcar or gondola immediately before the caboose).
I don't think anyone can actually say how good (or strange) it will look until someone tries it.
Andy
