As you suspect the size difference from 1/285 to 1/300 *is* noticeable during a game. Whether anyone cares is a different matter.I happily mix 1/285 and 1/300 just not in the same unit.
I too happily mix 1/285 and 1/300. But I mix them even in the same unit, and sometimes even on the same stand. Not always, mind you, and I don't suggest that even I would mix them on the stand or in the unit without some examination of the models in question.
But mix 'em I do. And it is not because I don't care how my 6mm armies look. Quite the contrary, I do care, very much. But among the major vendors I don't find universally applicable rules of mixable vs. not-mixable. And a nice paint job dominates any minor differences.
I thought I might put some images up on this very matter.
This is a pic of some AT guns from my 6mm WW2 Romanian force. In this one pic you can see vehicles (SdKfz 15 and Laffley W15 prime movers) from GHQ, guns (Pak97/38s) from C-in-C, infantry (rifle-armed battery HQ and LMG security section) from GHQ, and kneeling gun crews (older models) from H&R.
Here is my Romanian gun line. 75mm horse-drawn field guns are H&R, security section (LMG) is GHQ, standing gun crews are a mix of GHQ and (older) H&R artillery figures, and the HQ stand is one (older) H&R standing figure, 1 GHQ standing figure, and one Scotia kneeling figure.
To my eye, the Scotia figures are too large. They claim to be 1/300, but I feel that the kneeling radio man on the HQ base is too big compared even to the standing GHQ figure (1/285). That said, I don't find a lot of sources of kneeling figures with radios (or field telephones), and so I took what I could get.
To my eye the differences between GHQ and H&R are noticeable, but not so great that I fear mixing them even on the same stand. If you look at the artillery crews, in the third and fourth guns from the bottom you will observe a figure standing on the left side of the gun, a GHQ artillery crew figure, who is taller than the figure standing at the back of the gun, an H&R artillery crew figure. I can see the difference in height, but I do not see it as greater than the differences in height I see in any normal group of actual 1-to-1 scale human beings. And I've never experienced a gamer criticizing my forces for it.
I mention that the H&R figures are older, because I have just received my first order of some of the newer H&R figures, in this case some Italian figures, and they are really very nice figures. Substantially more detailed than the older H&R figures, still nicely proportioned, in some very good stances, and still seemingly pretty robust (meaning non-fragile, not meaning bulky). Size-wise they appear to be somewhat larger than the older H&R figures, but perhaps not quite as large as the GHQ figures. All based only on first observations -- haven't done more than open them up and looked at them so far.
-Mark
(aka: Mk 1)