I was only 18 months old when steam disappeared from the Southern region so I don't remember any steam on the main line at all untill it was preserved.
Whilst I can see that the railway scene was not good in the late 60s I don't see that as a reason not to model it. There seems to be a fashion to weather locos at the moment and they make fantastic models when done well, but they are hardly portraying the best times of the railway.
One of the best ever layouts I have ever seen was Dresden just after we British had done our best to destroy it in WW2. There were bomb craters everywhere, even in the track, with houses and other buildings half demolished. The layout depicted a scene of real carnage in a very depressing time for anyone living through the time. Having never been to Germany, I can't say weather it was an accurate model or not (although I'm sure it was) but overall it looked right in my eyes.
When I first saw this topic I tried mixing a bit of Blue/Grey with Maroon stock with a variety of both Steam and Diesel locos as seen in the late 60s. My very first impression was that it just didn't look right.
I think that's why mixed rakes aren't seen very much in just the same way that, for instance, model GNER HSTs are not seen with the odd Inter-city coach mixed in to the rake. It just doesn't look right not even in real life.