I know, when the Class 66 was initially produced, the Aristo-Craft intermodals were - and still remain - the only products made to complement the loco. The Bachmann Big Hauler three bay hoppers (1:22.5) seem to look quite well behind a Class 66, a couple of folks on this Forum have them. If you can get the DATA only bauxite versions so much the better.
PIKO have introduced a range of American outline cars in recent times. They are not highly detailed but are 1:32 and I am sure could fit well being a smaller scale.. 1:29 USA Trains might appear slightly out of scale with the Class 66 as the Class 66 is a British outline loco and therefore smaller than most of the products made in 1:29. I don't have a Class 66 so cannot make a 1:29 comparison. The Class 66 was produced solely for the UK market, although one or two Stateside dealers eventually stocked them.