I did a project some years back and we did mix 1:400 and 1:500 scale planes. It was for an airport diorama and had nothing to do with T Gauge. What we discovered is that smaller planes (regional) at 1:400 scale many times looked more correct combined with larger planes like a 777 at 1:500 scale. The perception of scale also varied based on where we placed the planes on the diorama etc.
The main â€œfocus pointâ€ determining scale for a plane is the windows. The reason the 1:400 scale Regional Jets looked better at larger scale is that plane windows are fairly small, 23 x 32cm, but when you sit inside a plane you donâ€™t feel the window being that small so your reference point is skewed.
A typical Regional Jet is 38m, at scale (Regional jets are between 30 and 45m, so that gives you some margin):
1:400 = 9.5 cm
1:450 = 8.44 cm
1:500= 7.6 cm
The difference in scale size for a window 23 x 32 cm.
1.400 = 0.575 x 0.8 mm
1.450 = 0.51 x 0.71
1:500 = 0.46mm x 0.64
A larger long-haul flight like a Boeing 777 is about 65 - 75m
1:400 = 16 - 17 cm
1:450 = 14 - 15 cm
1:500= 13 -14 cm
Windows are larger on a 777 about 25 x 38cm
My inclination is that I think a smaller regional jet at 1:400 scale will look better on a T Gauge layout. I base that on the fact that you perceive windows being larger than they are and that in general all planes are â€œbigâ€. On these smaller scales many details are a bit exaggerated in general so that is in-line with a bit larger scale.
If you use a 1:500 scale I think it may look too tiny, just based on the human perception of the size of a plane. But I have no idea how accurate to scale the planes you look at are and maybe windows are already exaggerated etc. The main criteria from my perspective is to determine if the windows â€œfeelsâ€ right.
Otherwise planes has the same general structural design and most of the time itâ€™s just the size of the plane that determines the size of the structural elements but windows typically are the same size.
Sorry, very long answer and of course this is my personal view