Manufacturers like LGB/USAT/Aristocraft/Piko & Bachmann's Big Hauler range tend to make their standard gauge US outline offerings around the 1:29 scale, save a little jiggery pokery on proportions to get them to go round tight curves. Accucraft's premium "Mainline" range is scaled at 1:29 too. When depicting US outline narrow gauge locos and stock, 2ft - 3ft in real life, they tend to go for something around 1:22.5. Big difference when matching things up.
There are a couple of exceptions to the above "rules" - Bachmann and Accucraft use the larger 1:20.3 (F) scale for their 3 ft gauge based "Spectrum" & "AMS" narrow gauge ranges respectively. And MTH and Accucraft, with their AMS branded range, use the smaller 1:32 scale (Gauge 1) for their US standard gauge items compared to the others. Some items out there are 1:24 too, e.g the USAT work train series. But the above all runs on the LGB type 45mm gauged track.
As mentioned above, you need to standardize on a coupler type. The only ones, knuckle that is, that gave me trouble working together with all the others were from the Bachmann Big Hauler range. Hook and loop are very forgiving but never look right to my eyes. Somewhere down the line though, if you decide to go with the knuckle type, you might want to look at the Kadee coupler make. You can standardize on them throughout or use ad hoc to iron out any anomalies you encounter. Well that's what I did. Max