Physical Limitations

So, the curves and physical limitations are going to be your key factor here.

If he can't physically run anything over 75', then the big 85-89' flats are simply out of consideration. You'll need to stick to the older 40-50' converted* flats. You're just going to have some leftover deck space loading a 35' trailer on a 40/50' flat.

*(The last CP car you posted in red, isn't a conversion but a new-built 56' single-trailer flat. But no one makes a model of that type. I think it was McKeen that did a 50' TOFC car that can stand in for this years ago, but those McKeen kits can be a) hard to find andb) a bit of a bear to build.)

Standardization

While personally I'm a but of a prototype purist for my own modelling, there's nothing wrong with running what he likes. It will look a bit anachronistic, but if he doesn't care and it makes him happy...

The heavyweight passenger cars and short TOFC flatcars would sort of tend to put an approximate time frame into the sixties (and the 48' trailers would be newer/outside that), but that also depends on what other cars and paint schemes you're running.

The only philosophy I'd "force" onto anyone else, is that taking the time and care to make sure things RUN reliably is going to be your biggest factor in being happy with something.

40 and 50-foot flats

Several roads converted short flat cars and cut-down boxcars into pig flats in the '50s and '60s, and then several more did it in the '80s.  Rock Island even converted War Emergency gondolas.  Con-Cor and Walthers have offered their 53-foot flats with trailer hitches.  Rapido will soon be releasing their model of the Pennsy F30 flat, and among the variants will be the Trailer Train version with trailer hitch added.

So before about 1965 there were several railroads running fleets of short flats.  After about 1965 most large roads had joined Trailer Train and were using TT's long flats, and/or bought their own.

Pup trailers never went away,  although they were not common on most railroads' piggyback service in the '70s and '80s.  (I think Santa Fe and UP hauled the most pups back then, but they were a small fraction of total loads.)  35-foot trailers were pretty much gone from revenue service by the mid '60s, and 48-foot trailers didn't appear on the rails until the mid '80s,  so no, you wouldn't see those together. 

What would I do?  Pick a narrow timeframe and stick to it.   It saves you a lot of money in the long run....

Scott Chatfield 

He said he doesn't care...

I am getting the idea that my buddy is not going to be much of an operations guy, moreover a model railroad railfan. Personally, I like both options, but this is for him and not for me. Although I am a late 1990s - 2010s modeler, he is willing to run whatever makes him happy. I am totally okay with that. He recently lost his mom and I think playing trains was probably good therapy for him. I know his wife appreciates the time we spend with his trains. 

Today, instead of working on my layout, I showed him how to upgrade an old TYCO flat car. He was rather impressed when he could pull a string of them around the layout and also back them up without a derailment ! Sure, we put Accurail Bettendorf trucks, Walther's 33" nickle plated wheels and body mounted kadees, but it took a cheap car and made it just a little bit better. I explained how they match an SP prototype, but he is okay with that terrible pea green color and Santa Fe markings. Okay, whatever...

Slowly, I am getting him to learn that little tweaks "here and there" lead to a trouble free layout.