Historically, Lionel played fast and loose with scale. I seem to remember that someone measured a 6464 boxcar and found it about 1:53 scale; likewise the "Santa Fe" type Hudson (2065, 665, etc.). Both were released in 1953, go figure. And some rolling stock is significantly smaller. Meanwhile, the GP7s, FM Train Masters, and F3s were all close to 1:48, as were the "chemical" tank cars and quad hoppers. The 2-bay hoppers are modeled from a much smaller prototype and are not so far off, but they are much too narrow. And many steam locomotives were designed to no scale at all. It's a mess! All this tooling was reused by MPC, and some is still in use in the modern traditional line. Other manufacturers (notably MTH and K-Line) designed some of their product to more or less fit in with this mixed bag of Lionel offerings.
So, basically, there are no rules in "traditional" because manufacturers did not abide by any rules when creating the trains. The only real criterion is whether you like the way it looks.
One thing I do, if I feel the locomotive is a little too tall, compared with the consist, is to use a low-sided car (gondola or flat) at the head of the train. It provides some visual separation, so that the difference in size is not noticeable.
Another thing I do is page through old railroad photography books until I find something that looks ridiculous (an enormous locomotive pulling a tiny cars, or vice-versa). Then I say, "there's a prototype for everything!" and congratulate myself on how realistic my toy trains are. 
