It could be done realisticaly. And why not? Russian pilots flew against US pilots in Korea, Spetz-Naz was lurking about in Vietnam testing out the SVD rifle in combat conditions, in 1991 Russian military advisors narrowly missed getting mixed up in operation Desert Storm.
And then there are the case of US and EU "advisors" mucking about in Vietnam, The Balkins, South America, and Afganistan.
Like how CM divided up sides by nations and then force types (Infantry, Armor, Mech, ect.) A "Russian Advisor" Nationality tab would not be out of place on the Syrian side. The force mix would be mostly infantry armed with High tech Russian toys and local 4X4 vehicles. Kind of hard to smuggle a T-90 in diplomatic bags.
Heck, for the purists, just add on a "Allow Foreign Advisors" switch for quick battles.