In HO scale I liked long trains and lots of units I mixed them all the time withou any major problems. I always tested the units to see which was the fastest, it would become the lead unit. The next fastest unit next and so on, I would run 4 or 5 units pulling 60 to 90 cars with no problems at all. (It broke my heart to tear down that layout). In many cases I reversed the units to see what would happen, but There were really no problems with that either. When running light with slowest units first the consist would sometimes bunch up a bit, but not enough to derail the units. With fastest first the consist would stretch out. I could start the units super slow with a momentum throttle and actually take the slack out of a 70 car train equipped with Kadee's. I do miss that layout/DesktopModules/NTForums/themes/mls/emoticons/crying.gif!! All I have left is the video.
I would let the units run by me at eye level at various speeds to see if any of the units were slipping their wheels excessively and if not would jot down the loco consist for duplication in the future. I wouls sometimes swap shells to get say all UP units in the consist or all C&K (Colorado & Kansas, my made up rr name) units in the consist.
I did it for years and years and never burnt out a motor. I used to run them on a modular layout my club operated and they would run for hours and hours at a time again with no problems.
I would think with care, my experience could be duplicated in large scale. I have different G Bachmann Steam engines which I double head without any problems. I might have to try tripple heading now that this post has stirred me all up again!!!
Have fun and try it!!!