NS did not MU the Baldwins and EMDs, although it wasn't uncommon the see a bunch of EMDs (or a bunch of Baldwins) with a couple of the other manufacturer's locomotives mixed in. NS actually had some pretty decent grades, and seeing a train with four or five locomotives on the front was very common.
As far as MUing, the only line I can think of that had Baldwins that could MU with other locomotives was Seaboard. Somewhere I have a photo of a Centipede and an RS3.
