I tend to recall a passage in the Rainbow Route about Otto Mears have trouble with the D&RG happily taking his cars but not being all that interested in supplying cars to him. Of course, the Silverton usually had D&RG cars and locomotives supplementing its own equipment back in its brief glory years.
The EBT cars haven't been interchanged...they've been sold for ballast purposes.
There are plenty of pictures of DSP&P and CC cars being interchanged. I can't recall if any of the photos predate Gould's take over of the South Park. There's also ample evidence of cars from the Kansas Central, Utah & Northern, and such being in service in Colorado. (the best photo of this is derailed U&N dump gons just outside Como)
When was the last D&RG(w)ng connection with the C&S cut (or rather, when was the third rail out of Denver on the D&RGW removed)? Interchange after that point in time would have been difficult since no 3' connection existed. I thought there was a bit in a book somewhere of a South Park freight car ending up in Mexico and being returned...but I can't think of how it could have gotten there.
The F&CC, in photos, seemed to use more D&RG freight cars than their own fleet of 300+ freight cars.
Michael