My HO layout is almost "too" specific in what I model, and one of the reasons why I am switching to O gauge is so that I can have more freedom to run what I want.
You are on a slippery slope, one a lot of us have been on as well LOL!
I can hear myself saying those exact words back in late 1980s.
I got into O-scale because I liked the size, but then I tried to keep RR specific, in my case running equipment that Seaboard Air Line operated. I've loosened that up a bit and also have some Atlantic Coast Line engines.
The prototypical bug bit but then I realized everything doesn't have to be perfect. I've modified, kitbashed, and built or had built a lot of equipment but I also have non-scale, non-prototypical cars running on my layout. Some are exactly what SAL and ACL operated, others are close or not even close. I have a number of these shorty passenger cars, in the end it's either that or nothing at all. Fortunately companies like 3rd Rail/GGD have come thru and given us scale equipment like their Silver Meteor 6-car passenger train. An ACL train would be nice
Buy what you like and if you find something you like but want to change, have at it. Doesn't bother me one bit to hack into a piece that costs several hundred dollars to get what I want.