Changing an entire unit into "skirmish" formation in order to avoid artillery casualties is a more-or-less standard wargame convention that has no basis in reality. It might make for an interesting game, but without any real life examples, it's impossible to incorporate it into a simulation.
Problems like this only become apparent when you're attempting to simulate an actual battle. In a game, everything is abstracted and conceptualized, and you don't have to think too much about how it was actually done when you're designing the game. When you try to model the reality, rather than just abstract it, you soon find out that some things that seemed obvious before have no way to be modeled. When you have to figure out how things actually worked, you find out they didn't always work the way you always thought.
For example, in TONG (This Other Napoleonic Game, in my case Austerlitz: Napoleon's Greatest Victory), if I had a unit under artillery fire, I'd change them to skirmish formation (actually in this case more like "open order"). They'd double their unit frontage. They took fewer casualties, and usually the artillery would shift fire to a more inviting target.
Let's say I was writing a Napoleonic simulation and wanted to include this. The problem comes in when I start reviewing drill manuals from the period and can't find any instances of how the troops were actually arranged, so I can't determine the actual unit footprint (width and depth) or how to move my troops to get into this new formation. Simply doubling the interval between troops would have little effect anyway, especially with enfilade fire coming from anywhere other than directly to the front. Also, I start trying to find examples from battle reports and there are none. Then, when I'm positioning my file closers (the sergeants who stood behind the formations to make sure no one left the formation) I find that I don't have enough to be effective, and I lose some morale effect from being shoulder to shoulder in formation. Troops will start running away.
The above is not a frivolous example. I was actually designing such a game at one time. What JMM has come up with is very close to my design.
So what were the light battalions used for? They were sent places that were unsuitable for formed troops, like clearing woods or attacking a town. Why were the light troops special? They could be counted on to act independently and not run away. Generally they were the only troops trained to use aimed fire.
I'll defer to Count von Csollich on how the Austrians actually used their light troops. Generally they'd be use for typical light troop duties as above, and in modern times some would be broken into companies and attached to individual battalions if skirmisher screens were deemed necessary.
Hook