I have a pretty big layout, main line curves are 128. These days I only buy full scale length cars, but have trains with 18" cars which I enjoy running. I even have two trains by K-Line which are 15 inch length but otherwise scale - they look great and I enjoy running them. My only rule is I don't mix coach lengths in the same train.
I designed my layout in the era when 15" cars dominated. My assumption was passenger trains would be 8-9 feet long maximum. My central terminal is on a loop with four through tracks, each sized as described. I now often run trains of 20 inch coaches exceeding 12 feet. My carefully designed terminal can only handle one 12 footer at a time... I also run shorter local trains... LOL
Jan