"There’s debate on whether 1/35th scale figures can be used around 1/32 scale aircraft or vehicles. It’s for each modeller to decide but I find the scale difference barely noticeable – mindful that the average height of combatants in WW1 was apparently shorter than people today.
There’s also the trick of visual perspective- place the 1/35th scale figure slightly to the back in a diorama and it appears as if the figure is a bit further away which actually lends depth to the scene."
I have been a full-time professional figure sculptor for over 35 years now. I have been a model builder for even longer than that. As a figure sculptor, making figures that go together with models has always been my forte. The mixing of scales by modelers has been going on forever. I suppose if you are a causal modeler, then such considerations may not be an issue. But I have always strived for "realism" in both models and figures. To my mind, realism entails a mindfulness to relative sizes and scales, among many other factors.
As for figure scales and relative sizes, manufacturers are all over the map. I do figures in both 1/35th and 1/32nd scales. The relative sizes of the figures in these scales is, indeed, not greatly differentiated. I take 50mm as an "average" size for a man in 1/35th scale. For 54mm I take 54mm as my average. Indeed, I do make figures shorter and taller than these sizes, so these are rough guidelines. Many manufacturers use different "averages" for their figures. For instance, my 1/32nd scale Model Cellar figures do not work with those of Kellerkind. The Kellerkind figures average about 5 feet to 5 feet, six inches in 1/32nd scale. My figures average about 5' 10" in scale. They look ridiculous together. Furthermore, my heads are a bit larger. So, even within scales, compatibility is tenuous at best.
When you start adding in different scales to this, the differences can be quite extreme. The things that most clearly indicate figure scale are heads, and equipment. Heads do vary in size among individuals. But overall, they vary much less than say, the overall height of a figure. With equipment, well, it is in scale or it isn't. These sizes should not vary at all (even though they frequently do!).
Caveat emptor. If you are into realism at all, or scale conscious like me (as I must be in my profession), then mixing of scales should really be avoided. I am sorry, a 1/35th scale support vehicle looks ridiculous with a 1/32nd scale airplane. It is too small and noticeably so. With figures this can vary more, either in your favor or against it. But, I feel, it is always best to stay conscious of scales and sizes. Realism demands it. That is my opinion at any rate....
As for forced perspective, I have seen this used very effectively, especially in box dioramas where the viewing angles are carefully controlled. In an open diorama, not so much. Building a diorama with smaller scale models "in the distance" can look pretty good from the front of the diorama. But turn the base around and the thing just looks ridiculous from the back.....
Mike