Great site! Keep up the good work and thanks to you and TLA-ers for the great job you do in promoting the hobby - NOT for competition, but for the love of painting, playing and history!
Do not change the rules regarding Cossacks charging formed cavalry! They did so numerous times and mauled the Polish on several occasions on the advance into Russia (for example).
Since their primary role for some time was fighting and maintaining the southern frontiers of Russia against the Turk and the Persians, they had a radically different style of warfare than the "Westerners". Also, the Cossack (in most all his various cultural/racial forms) believed that to survive a fight (to terrorize ones enemies later and to survive to care for one's family) was better than to die foolishly for empty "glory". But, if ordered to do so, they would charge with the ferocity of the finest Europe had to offer!
To reflect their style of warfare, have them take a test at a reduction (slight, say -1 - if your rules use that) if charging FRESH (in good morale, order and having taken no reduction in numbers) enemy from the FRONT. Otherwise they'd have no reduction.
They were good cavalry, contrary to what most Western authors (who understand/understood little of their lives and style of warfare) might write. Even Napoleon himself said they were the finest light cavalry in the world!
I hope this post encourages you to re-think the change mentioned!