Austria has the only "light infantry" unit in the game. The U.K. 15th, the French Voltigeurs, and the Hessen-Darmstadt units are all skirmishers who cannot spawn with a rifle. The only different is the weapon. The Austrian Grenzers have different stats; their polearm and shooting proficiencies are pretty nearly halfway between those of line units and skirmishers. The only two units Austria doesn't have are rockets and heavy cav (heavy cav and cuirassiers have different stats - Rheinbund and France are the only factions with both).
Take that as you will, it's easier to just call it a skirm if it's green or gray and a light infantry if it's got a bayonet.