Also, just one idea with the Russians/Volga Bulgarians which I think is pretty cool. Most of the wars fought between them were not so much for territorial aquisition, but for influence over the minor tribes. To represent this, you could have about half a dozen units that can only be built in a couple of provinces, like the Spartan Hoplites in RTW. CA did this by having spartan troops requiring an invisible resource, called 'Sparta'. You will need to have a building that can be built by Novogrod, Kievan Rus and the Volga Bulgarians, maybe the Cumans too. The units would add something to the army list, and would be better than just mercenaries, as anybody can just wander in and hire them, and it is more realistic that only the Steppe factions can build them. Also, this would mean that not so many units are required in those factions army lists, and so frees up some unit space. I can supply the names and descriptions of the tribal troops, but from your XL mod there would be the Bashkorts, buildable in the Volga Bulgarian home provinces, and the Chernye Klobuki, buildable in the Kievan Rus home provinces (although they were historically much more widespread). I have some others, but still finalising information for them.
While on the subject of the Rus, here is a website describing arms and armour that might be useful to you.