From different "manufacturers", different sizes of uniforms:
I've got one like that. The guy on on the left is of dubious origin, if you can believe my DNA results anyway, but mainly Scottish and Far East. Then we have German, Japanese, Vietnamese, Korean, German again, and Australian.
But the one thing that remains fairly consistent is head size.
If you think that the problem in 1/35th scale is bad, check out 1/48th. From right to left: Monogram 1/48 from the 70's. Allowing for his soles and helmet, he scales out to just about 6 feet tall. That seems fitting for a corn fed US paratrooper. The middle German is Tamiya, circa 2006, and scales out to just over five feet tall. Clearly not the genetic Uebermensch we might expect. And the US infantryman on the left, while not standing, is still childlike in comparison. He is circa 2005. Perhaps his parents were vegan.
And yet, when we look at equipment, the Bandai M1 and Tamiya M1 are virtually identical in size, although Tamiya wins slightly in detail. Since the Bandai figures are larger than Tamiya, you'd expect their helmets to be larger, but they are not. Tamiya's is ever so slightly larger than Bandia's, especially width wise.
So what's it all mean? The kneeling figure definitely gets posted farther back than the standing Monogram figures, as I alluded to earlier. The Bandai figures are just donor figures for body parts. The child sized bareheaded German figures face the guillotine - not much else is useful on them. They'll receive Mohawks.
But as for mixing the equipment, the Tamiya equipment looks about right on the Monogram figures, except obviously the helmets. But from three inches away (twelve scale feet) even the helmet belonging to the helmetless paratrooper looks fine.
So just be careful in your arrangement, and it'll all work out.