Besides scale, you should be looking at the era you are modelling.
First off, the oh so common Aristo “40 foot box car” as built, was equipped with a steel underframe, and modern freight trucks…some that lasted to the end (And you would be hard pressed to find a 40 foot box car in service, let alone approved for “Interchange” after the late 50’s). of their interchange lives, a few may have been converted to roller bearings.
The US Trains “American Series” cars, which are/were the same scale as the LGB reefers, and box cars, whatever scale they were. They were models of narrow gauge rolling stock, with wooden underframes, and truss rods. They had draft gear that would not be approved for interchange after the early 40’s, if then. They also were produced with arch bar trucks which also were outlawed soon after or earlier than the mid 40’s.
The brake systems were also different…the older brake system was not approved for interchange at a certain time , possibly in the early 40’s.
There is more to model railroading than just "Do they look good together’, IF ANYONE CARES ENOUGH to actually research or care about the railroad transportation industry, that we supposedly are modelling.
So; it could be stated, generally speaking, that it is not appropriate to mix the two types of cars.
But…on YOUR Railroad…do what in hell you like…!!!..and have fun, at all costs.
Fred Mills