Here's what I have written on the French Old Guard which covers some of what you're talking about. Generally Guard units might be actual guard units but in reality they were often "elite" units that were better trained and better equipped. Napoleon used his Guard as a decisive reserve when needed (such as at Marengo when they were used to hold the line until Desaix's division arrived on the battlefield) or at Waterloo (when the Prussians were flanking).