Gentlemen,
Forgive my late reply to this topic which I find I need to add some perspective. As most have jumped on their high horse (or pony as the Cossack case may be!) and missed the whole point!
The point is a group of skirmishing cavalry! Not formed and supported cavalry groups!
I completely agree with Christian and Anthony's explanation and rule in regards to assuming any LIGHT CAVALRY unit of less than 100 can and should be considered under the skirmish rule. I in fact like the idea of removing the 100 limit in regard to any infantry skirmish unit as well, but that's another point lol.
I too have done extensive research and reading on the period and can assure everyone that this rule is not trying to belittle or reduce the 'manliness' and effect the Cossacks had in reality. However besides the Guard Cossacks, Cossacks were not battle field main units unless in large, grouped and supported numbers. When operating in even Regimental strength/groups they USUALLY avoided contact with FORMED/ORGANISED units - unless they had overwhelming numbers, caught the enemy by surprise or well supported by regular troops. This is a fact, the examples given by others above are also true facts, but are referring to large force/groups operating as such, NOT skirmishing.
This is NOT what Christain and Anthony are on about. They are on about the use of Cossacks (an all light cavalry)in skirmishing mode.
There is not a valid argument presented above not to put this rule in place for all such Light cavalry other than maybe to reduce it for Regular light cavalry to 50 troopers but would consider 100 Cossacks in the same 'skirmish' rule.
I might add that all historical accounts should be taken with a pinch of reality in regards to the sources used, especially when from only one aspect. I have found personal memoirs and multiple unsolicited accounts of similar situations, experiences and actions from a cross section of nationalities involved give a much better perspective and understanding of the actual realities of units, personalities and actions.
Light cavalry operating unsupported in troop size units or in understrength squadron of less than 100 do have an unhistorical and unfair advantage in relation to how they can stop large scale/force movement and movement in obstructed hexes within the game. They too operated as the Cossacks did and would not engage main force units! This is able to be countered as explained above but still does have a disproportionately unhistorical impact on the games. The more Cossack units you have able to roam in small groups the more disproportionate effect it has. The rule as suggested I feel handles the situation as best the game and players can cope with.
I really do consider Christian and Anthony's arguments and reasoning as being sensible, historical and practical in all regards on this matter. The counter arguments are not countering the SKIRMISHING LT CAV issue!
I fully support the interpretation and will use it in my future battles against all such situations when playing MOE and Embedded Melee type rules.
NWC
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, However, I listened to his arguments and I've found that our new rule (which considers them as skirmishers) does seem to make you use them in a different, perhaps more historical, way. You can still exert pressure with them by keeping at a distance but working your way around the flanks and threatening the enemy's rear, or dominating the road network, or other similarly irritating roles 
. All this ties up the enemy's light cavalry, as the cossacks can soon be made to descend on weaker units in a swarm, where the skirmisher exception to the embedded melee rule wouldn't have much effect due to the number of cossacks involved relative to the strength of the defenders.
, want) is to avoid their defensive deployement as if they were simple skirmishers! Which is, I wish so, an unhistorical fact that everybody can agree. No way for us to forbid them any gallant deed! 