It'll come as good new to modellers to know that even in steam days, "Enterprise" formations could vary.
It was only with the introduction of the NIR Mk. 2 stock in 1970 and the "Supertrains" in 1972 that the one-type-of-coach fixed formation idea which is 100% the case these days, ever came in. Prior to that, passengaer train make up was based on one thing and one thing only - whatever was available, and most suitable. Unlike the severely retrograde situation today, where no two types of vehicles can even be coupled together, ALL railway vehicles had the same couplings, and no electrics were involved involving complicated and non-compatible jumper cables from vehicle to vehicle. thus, ANY vehicles could be coupled.
I have seen a photo of an AEC railcar set about to leave Harcourt Street for Bray, with a 1890s MGWR six-wheeler towed at the back. Centre cars for AEC railcars sets, even, could be ex-GNR, ex-GSWR and ex-NCC former steam stock, with minor modifications for gangways etc.
Right into the mid 1980s when the last wooden stock disappeared, a train make up on Dundalk to Bray, for example, might be BR Van, laminate type 1, Craven, Park Royal, laminate type 2, laminate dining car with shutters closed, Craven, Laminate brake standard genny. I have a photo somewhere of a Ballina-Limerick train about 1972 or so which has a 121 hauling a "Tin Van" followed by a laminate and a Park Royal.
I can remember being at Port Laoise about 1974-ish photographing the stub of the Mountmellick line down in the yard. The H & S police didn't exist, and at extreme threat to my life I wandered about a deserted yard freely, sans steel-capped boots, PTS or day glo knickers. The 1030 Heuston - Cork was heard arriving in the station, so I ran to gain a suitable place to take a photo as it left. It was 141 hauled, and behind it were some 10 bogie vehicles, with no more than two of the one type, a laminate brake at one end and a tin van at the other. In a mixed formation (which was absolutely the norm), there were examples of two or three types of laminates, a Craven or two, a wooden-framed dining car, and a Park Royal or two.
Go back ten years earlier and you have old wooden coaches too - almost all, if I remember correctly, which survived into the black'n'tan era being of GSWR origin. Bredins were still about too - the last time I saw or travelled in one myself (with a leaky roof!) was about 1975 or 6.
So mix away.
Conversely, a passenger train pre-1970 with a pristine rake of a single type of coach is actually not authentic at all 99% of the time! (If not 100!)
Regarding GOODS trains, it's 101.1% of the time!