The advice about not forming Gds brigades is driven by knowledge of the combat engine. Part of this is it selects targets and sets up brigades (and loose battalions) for combat. The largest (in terms of combat power) is selected first so if your Gds brigade is in a standad corps it will be in combat.
Now it will win, most likely, but its almost bound to take heavy losses.
If you want to create this sort of brigade put it in the stack with the army commander. This is almost always committed in support of a corps already in combat - so by the time it arrives the enemy is engaged. Can be an effective way to overwhelm the enemy.
The map in RoP doesn't really reward light forces on their own. Its not like WiA where raiding and skirmishing is part of the core game. Part of the problem is both sides have fairly large armies and if a light infantry brigade comes up against regulars it will tend to lose. I now think the best use of the Austrian Grenzler battalions is outside the brigades and loose in the corps stacks (as above they do not cost command points so this is efficient). Also light infantry as in-formation skirmishers is more Napoleonic than a feature of the Seven Years War.
While this is all for Wars of Napoleon, some posts here will help: http://www.matrixgames.com/forums/tt.asp?forumid=1569
as the routines of stack and target selection are identical and may help you with how to structure your brigades