The distinction between Soviet and American service branches is totally fair - I have no real qualms about the way it's modeled! They're just two very different doctrines and models of command and procurement. The simplest explanation is that the US economy can afford to have multiple services with separate equipment standards and requirements (which have remained despite all efforts to unify and standardize them) - and the Soviet economy never could. Rather than truly different services, the Soviet branches like airborne are best looked at as special-purpose units within a common system.
I kind of had some similar thoughts when working on the Chechnya mod - initially, I was considering separating Russian Army, Airborne, and Interior troops. There are some differences between them - only the airborne forces use BMDs, and for example only the interior troops were using T-62s in that conflict.
But after mulling over it, I realized that it wasn't a good depiction of how they actually operated. Because ultimately, the Russian/Soviet approach treats them as pieces of the same machine. One reason for this is that the Soviet approach has always been (by necessity) top-heavy - with high-level, central officers, high commands, and their staffs always overruling lower-level commanders on the ground - and political nomenclature overruling military rank. So, whatever branches there were - didn't really matter, since they would have very little independence in practice.
As for the USMC - well, look on the positive side, it'd make a great package with a DLC that could introduce amphibious features