Ernst,
Looking through Osprey's "Frederick The Great's Army 2 Infantry" I noticed NO
bayonet scabbards worn by Prussian soldiers on any illustrations. Be careful by adding it next to the "hanger" unless you have reliable reference. The bayonets are very short and were more than likely fixed all the time. I'm not an expert on the Prussian Army during the Seven Years War but I've never seen an illustration/painting depicting them without bayonets fixed. This practice, as Gary mentioned, was followed thru to the Franco-Prussian War of 1870-71.
As far as the cartridge box, they were very large. I would guess twice the size the Napoleonic era Prussian boxes.
Keep us posted on your project.
Jason