Unfortunately there is little (if any) compatibility between manufacturers, sometimes even between their own lines of product. Trucks and how they fit onto the rolling stocks pivot point would be one of the most frustrating. Axle lengths vary widely. One company, Reboxx, make metal wheelsets with metal axles in a large variety of axle lengths, But of course you need to be able to measure your axles from tip to tip to know what to order.
When it comes to couplers the industry/hobby standard is Kadee. In the past their #5 was the recommended replacement for the vast majority of factory fitted couplers. These are a "Knuckle" type, mimicking a prototype (i.e. real coupler) in appearance and operation. They consist of the coupler itself and a brass centering spring that fits into the coupler box on the car. They fit most model manufactures cars. Bachman, for reasons known to themselves, made their boxes a fraction (hairs-breadth) smaller, and won't take the spring. The new Kadee #148 (has the same size coupler head as the #5) or the #158 (smaller semi scale size head) have done away with the separate brass spring and incorporated it into the coupler itself. They are called a "whisker" coupler. Either of these will fit into Bachmann boxes as well as most others (don't know of any they won't) So, I would suggest standardising your fleet with either of the #148 or #158. The #148 is more tolerant of misalignments in coupler height or track bumps and dips because of it's overscale size and are a bit easier to uncouple manually using a pointed stick or Kadee's coupler tool. The #158's just look a bit nicer.