The older AM freight cars tooled up in the 80's have slightly less detail than SHS, but are fine. It doesn't take much to upgrade them. Most of the boxcars in the previous photo backgrounds are AM's.
The practical minimum for scale operation I would place at 27" radius. That's about equivalent to 18" radius in HO.
I was using 33" and 29" radius on my old layout. It still allowed me to operate AM's 85' scale passenger cars with body mounted couplers.
I also has a 27" radius on a siding for testing purposes.