If you look at pictures/videos of steam era trains, say in the 40s and wartime traffic,
you will notice there are many different sized cars of all types. In that period in particular, and before/after, railroads used cars of all sizes, the requirement dictated by the cargo associated and availability. I mix different sizes all the time, and as stated above, it looks fine. The only factor to look for concerns car weight and placement in the consist, length of consist, and correct placement to eliminate problems of lighter cars pulled over while negotiating curves.
