Hey Matt
THE best narrow gauge cars are by Accucraft, but they are also WAY expensive… all Colorado based on real prototype cars. they are to die for if you can afford them.
Bachmann also makes very good narrow gauge 20’ and 40’ frieght cars, cabooses and passenger cars, all at very good prices, they are mostly Colorado prototype based cars although they claim some East Broad Top cars. you can also look on ebay for older boxcars, tank cars, stock cars, and kits.
LGB also makes some narrow gauge item but they are at LGB prices too. Very good quality-very high price. The frieght cars are Colorado based, as are the larger wooden passenger cars. They also have shorty cars that are not specific to any prototype, but they have the best selection of cabooses IMO.
Hartland still has some good cars, there Sierra type short passenger car is really nice, its at 1:24 scale, but next to a 1:22.5 engine Iooks fine, but the long passenger car looks like its 1:29 scale next to it. their freight cars look pretty good but are slim on details.
As for couplers and auto uncoupling? I think your going to be looking at LGB which has an remote uncoupling system, though I dont know if it works with there knuckle couplers, I would imagine it would. As for Kadee i havent used there large scale couplers so i dont know if they have an uncoupling system. Bachmann doesnt, I know that.
LGB knuckle couplers are easy to mount to any other brand like Bachman as they are based on the “standard” LGB hook-n-loop mount.
Dont worry too much about “mixing” different scale freight cars. 1:20.3 next to 1:22.5 and so on, most real RR’s had such an ecclectic mix of cars seeing an odd mix of cars was common, dont believe me ? look through some books, you’ll see.
Good luck with it, Vic