To my knowledge the Chechens were far from magnificent and contributed to large portion of Russian casaulties by running blindly into a short-range ambush, but that's another matter... If you want Chechen forces added to your mix, buy them as "Allied" either from Chechen OOB (70) or Red OOB (43).
The AI will not do so, but then their use is fairly limited to few special occassions on Caucasus.
As for Russian infantry units, I won't touch the OOB with a ten-foot pole if I were you, it is just so interlocked on all sides that making a minor change might result in the need to rework all. Believe me - I have tried to change APCs from "Wheeled", "Tracked" etc to "squad-capacity" (BTR-60PB+ and BMP-1+) and "larger" (from early BTR's with cap of 17 to MT-LBs) and it was pretty painful
Add to that very few free slots...
If you really wanted to do some changes to Russian OOBs, you might use few tricks still (like changing all snipers to para snipers and deleting the duplicates etc) but you need to be very careful.