Old post, but just saw it and may still be useful to other folks. Keep in mind 15mm is not a "scale" it is a size. Lots of words on the web about scale, check the TMP thread here: TMP link
And here is a good site for size comparisons: https://brooklynwargaming.com/tag/15mm
And here is a good discussion from a model train perspective: thortrains.net/scale1.htm and here thortrains.net/scale2.html
Model train companies have long built stuff precisely to scale so that different manufacturers stuff all matches (and will actually run on an HO/TT/N/Z scale track). Toy soldier manufacturers have either never figured this out, never bothered with it, or actually intentionally make their stuff so that it won't mix with a competitor hoping you will buy all your stuff from them!
If the average human is 5' 8.5" tall and 15mm is the distance from bottom of feet to eyes, then 15mm equates to 5'4" = 64" = 1625.6mm so 15mm is 1/108.37 scale. If you say the average height of a human in 50BC is something less than 5' 8.5" tall then 15mm will be something different. So the ever popular 1/100 is only 8% off or thereabouts. I actually use a lot of TT scale model railroad vehicles and buildings for 15mm (that is 1/120 and is an old and increasingly hard to get scale manufactured mostly in the old East Germany – I used to wander into the Soviet Zone in Berlin to buy the stuff, usually in exchange for coffee, chocolate, nylons, or cigarettes back in the day :-) but I digress).
Using the above logic, 1/87 actually works out to 18.69mm, 1/72 is 22.58mm; 1/76 is 21.39mm, and 20mm is 1/81.28 scale. 20mm is noticeably smaller than 1/72 and slightly smaller than 1/76. Thus, 1/87 is 13% smaller
If you compare 1/72 and 1/76 scale, that is a 5.25% difference and yes, if you put two Shermans side by side you can tell the difference. 1/72 to HO (1/87 = lots of Roco stuff) that is a 17% or so difference and very noticeable. 1/72 to 1/100 is about a 25% difference so your 15mm stuff will look that much smaller than 1/72.
To make matters worse, different manufacturers put out different size stuff supposedly in the same scale. And some manufacturers actually change the scale on their box (!) – for example, Airfix put out some old troops originally in 1/76 scale, but when that scale sort of went out of fashion in the 1990s, they started putting those same 1/76 scale figures in a box that said they were 1/72!
As far as mixing scales on the table top I actually do it all the time (hiss, boo, heresy!!). I have a ton of HO buildings I took off my dad's train layout when he passed away and they work fine for either 1/72 or 15mm. With other stuff, the trick is not to mix different scales of the same TYPE vehicle i.e. make all your Shermans 1/72 and your Panzer IVs 15mm and many folks won't tell the difference. Especially true with trucks – who can really tell how big a deuce and a half should be next to a Sherman? I know some folks who play Saga using 1/72 figures (the standard scale for Saga is 25mm) and only the purist curmudgeons ever grouse about it (and then only if they lose with their 25s to 72s).
I have even been known to actually mix 1/72, 1/76, 1/87 AND 15mm on the same table at the same time – I ran a big North Africa battle with 72 Shermans and 48 Panzers (of varying types) and various scale infantry. It actually looked better when I ran the same troops in Normandy (except for the North Africa camo scheme) because all the trees and buildings broke up the battlefield and didn't make the scale difference so obvious. Nobody complained although some did comment on the scale difference ("Are these different scales??")
So the real question is do you want to have fun pushing toy soldiers across a table top battlefield or are you constructing a diorama for the British Museum? If the former, by all means mix scales. If the latter, you best stick to one scale or don't expect to be paid…
