I definitely think 10 is too low. Depending on R1, the Russisans can have (and in some cases need) 12-16 inf, 3 arm, 2 ftrs in kar at the end of R1.
I think a stack limit of 20 for all territories with capitals being 30 or something might work. I think 20 will safely cover (at various stages of the game) all territories except: Ger, EE, Kar, Ukr, Mos and Novo.
Also you could try to only limit inf. Say 15-20 inf per ter, but you could have as many arm, ftrs, etc as you want.