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All well trained Napoleonic regular Line Infantry could deploy to skirmish order.French, British and Prussian Line and Light troops were all equally capable of fighting in extended (skirmish) order or close order.No set of rules should prohibit Grenadiers from skirmishing, nor make it compulsory to deploy Voltigeurs in open order. How you base them is up to the rules you want to use.
For what it's worth, I intend to have two French combined grenadier battalions in my Peninsular War collection. I do not intend providing them with skirmish stands.Purists will wince at this but, that they could skirmish doesn't interest me. For me such units will only be used as 'elite close order assault' game counters. If I allowed them to do everything I would find every wargamer, playing the French, would field as many of these 'do everything', 'elite gaming counters' as they could - which the purist would also wince at.