From my experience, and what others have told me, TMCC or Legacy steam and diesels do not mate up in a lash-up. The gearing is totally different. I played around with a Legacy UP E7 and my Legacy 4-8-4. Never got them to sync.
What I do when building a lash-up, I place the engines on the track about 1 to 2 feet apart. Once the lash-up is made, I then run them to insure that both are in sync, and that one does not move faster or slower than the other.
If they all run in sync, then I know I can couple them up and run 'em.
RAY