I upset a rivet nut a few years ago. I was asked if I knew that I was running DR stock with DB and THAT was a French coach I was running with my Ziller coaches?! What is more I was running standard gauge American stock together with narrow gauge.
I knew about the DR/DB 'cos it said as much on the sides but both they and the French/Ziller were the same colours so, at a distance what was the problem? The only problem with the American stock from my point of view was that occasionally it decoupled.
I know little or nothing about the nuances of Continental or American railways but what I do know is that in Scale 1/1 reality narrow gauge mixes and matches all the time. Just Look at Welshpool and Llanfair for a start..