Not to be joy killer, but you can't really calculate that way. With the reduction of the strength from 18 to 15, the odds against, e.g. a rifleman, changes dramatically and you would need more than 120/90 = 1.333 times more cossacks to do the same amount of damage.
To put it another way, is a fight between a spearman and two warriors even, since the warriors' combined strength is the same as the spearman's? The answer is of course no, because of the way the combat mechanics of the game work.
edit: Found a calculator - the odds for a cossack against a rifleman with no defensive bonuses changes from 64% to 29% with the strength reduction.