I am generally of the opinion that its my model railroad and that I can run it as I like.
Anyway, not all types of equipment are available as models, so unless you have the time and skills to scratchbuild or modify your own, you sometimes have to accept compromises.
To me, if it looks vaguely credible, then it works.
Of course you can always invent a story to it. This car is being deadheaded, and that one is included because the regular one wasn't available today.
I sometimes run a train of phase V superliners (which I claim is the Southwest Chief) and have it pulled by Santa Fe warbonnet F-units. I like the aesthetics, but of course it's complete nosense. But them it's my railroad so I will do as I like.