Russian "little green men" aren't attached to ISIS, (or any Syrian insurgents for that matter) which I assume are the insurgents in this game. They're attached to pro-Assad forces which actively avoid Americans.
The Russians, mercs and pro-Assadists are fighting mostly Arab Sunni Islamists (though there are moderate Sunnis in Assad forces as well) and ISIS to the east(ish).
The Americans are attached to the SDF, who are also fighting ISIS, in their own little area to the Northeast they've declared autonomy in. The SDF (particularly the Kurds) sometimes get into spats with anti-Assad Sunni insurgents, but They want nothing to do with fighting Assad or the Russians.
So yeah, I guess you have "little green men" on insurgents, but it would be a departure from realism considering it would basically mean WW3 is going down. Would be like adding women insurgents. Wouldn't make much sense.
That being said, Chechens would make a whole lot of sense.