I would expect anybody who has reached this page to be aware of most of the common scale/gauge combinations that are commercially available to the railway modeller but, just in case you were not, or if you want to see some justification for the "correct scale for gauge 1" statement then here is a very quick outline.
Model railways are riddled with odd scale/gauge combinations and mixtures of metric and imperial measuring systems.
If we start with O gauge fine scale then one would expect a scale of 7mm on the model for every foot of the prototype. This gives a scale ratio of 1:43.54 (the connection between Hornby and Dinky may help to explain why one of the most common scales for diecasts vehicles is 1:43). The trouble starts when one multiplies the scale by the gauge and taking the accepted O gauge of 32mm one ends up with either 4ft 6in (using 1:43) or 4ft 7in (using 43.54). Half O would therefore be 3.5mm to the foot at 1:87, which apparently gave problems for the early manufacturers of british locomotive models who went for 4mm to the foot (1:76.2) on the same 16.5 mm gauge (and which in turn led to the call for more accurate standards such as "protofour"). If you are wondering why O gauge was not set at 33mm, so as to be in the same proportion as Ho, it may help to point out that 32 mm is just 10 thou (imperial) over an inch and a quarter. In the early days these "round numbers" may have helped a few people.
I do find some of the choices of the larger model/estate/amusement gauges to be equally odd. The ones that cause me most brain-ache are the 7.5" and 7.25" sizes, what scale are these supposed to be? If you care to do the maths consider the possibilities for 1:8 scale. I regards this as a very "sensible" scale (if you can cope with the costs and size) as after all, no matter what your measurement system is you just need to take a prototype dimension. halve it, halve it again and halve it for a third time to land up at the scale size and, unless you are considering 1:10 in metric measurements or 1:1, 1:2 or 1:4 scale in any dimensions the maths will not really get any simpler. If you consider the 4 foot 8 and a half inch standard gauge the maths for 1:8 scale is simple enough to do in your head. One eighth of 4 feet is 6 inches, one eight of 8 inches is another inch, one eighth of half an inch is one-sixteenth ; total 7 and 1/16 inches. So why on earth were 7.5 and 7.25 inches selected? These gauges were not after all adopted in the early "bare sixteenth or full thirty-second" pre-Whitworth era but followed much later. If someone can gently explain why these dick-headed gauge scales of 1:7.533 and 1:7.793 were selected I would be obliged!
As for gauge 1? Well regardless as to whether it is finescale standard gauge or "G scale" narrow gauge the rails are expected to be 45 mm apart. Needless to say there are gauge 1 enthusiasts who love the mixed metric/imperial system and in this instance (standard gauge) it is usually 10 mm to the foot. If one takes the 10 mm scale (1:30.48) one would end back up with a gauge of 4ft 6in. If one takes 1:32 scale then one ends up with a gauge of 4ft 8.7in, which is the most accurate result yet mentioned. Once upon a time some people took a simple imperial view on gauge 1, an inch and three-quarters being 44.45mm and on plain track I would imagine that most modern wheels will cope with that dimension, the likely problem would be with the back-to-back dimension and check-rails at points. I love my OO railway items but I would never claim that they were true scale models since the gauge is out by 12%. The same has to be said of 1/29, the models may have a lot of detail and/or capture the spirit of the original but (if representing standard gauge prototypes) they can not be true scale models.
Please understand that this is not intended to "flame" anyone who likes the other scale to gauge ratios. If you are currently running one of the G scale brands then that is fine by me, similarly if you want to mix scales on the same gauge as long as you are happy with the result. If there is a point that I wish to get across then I suppose it is this: If you aspire to the finer gauge 1 scale models then these will probably be to 1:32 scale. If you want the locomotives to look in proportion to the rolling stock then it makes sense to keep to the same scale and if you intend to "eventually" have 1:32 scale items then why not start out in that scale?
If your experience of american outline rolling stock has been the 1:29 scale Aristocraft or USA Trains models then you may find these images instructive (quick comparative dimensions of the MTH model boxcar gives a height of 142mm, rail to walkway and a width of 91mm versus 160mm and 105mm for the Aristocraft model). A final word if you do intend to mix scales, I would suggest that you put one or more items of rolling stock that do not reach full height (gondolas for instance) between boxcars to help make the jump in height and width less obvious.
1/32 scale M1a (now that is a decent finescale model) with an MTH boxcar, compare height of boxcar walkway with doghouse cupola on tender. To my eyes this does not look too stupid for height or width.
I'll concede that the MTH stainless wheels look too shiny compared to the plastic Aristocraft ones, a bit of paint would do it.
Another height comparison against the cab of an Aster K4
I suspect that you have got the idea by now