Yes, but, in this case, the 20 grains of upkeep per unit means you're not only providing the vehicles, but the troops with them, and putting them under a russian commander. That, of course, isn't the rule in WWI. Remember how GB and France suggested USA to integrate their troops into british and french armies, encountering with a clear negative response from the United States men, who wanted to fight as AEF. Finally, french, british, US mens, canadians, indians, ANZAC, etc., should fight according to teamwork under a unified command, with their own troops and weapons. Unit trading avoids to replicate that.
How about volunteers? I addressed literal boots on the ground, Canada, Poland, and USA is known to have done this in ww2. Or perhaps the Spanish Civil War volunteers, troops joined that from all over the world, though USA nor any other countries did not declare war on that from what I can tell. And under your argument, shouldn´t bytro be working on something similar then since, idk, you admit it happened historically?