You certainly can clump everything around Nappy, and he can then demolish anything he comes close to. I find that this just means any enemy army will (quite reasonably) run away. So now you'll have Austrians and Cossack having their merry way with your hinterland as you try to advance.
Besides, you'll have to besiege cities on your way to Vienna (e.g. Munich), which means you'll have to split off troops at some point whether you like it or not.
Being a noob myself, I've found that having my corps spread out in mutually supporting areas is much better. Just make sure that Napoleon is within support range of anyone about to do battle.
And please don't do what I did. Forgetting that you're invading enemy territory, and thus areas are liable to receive insurrection troops if you don't guard them.
As for Italy, you can't beat Karl in a stand-up fight without fortifications. He is simply too strong.
What you can attempt to do is get into Mantua before he arrives. That usually means throwing a blocking-force onto the road leading north-east, and having them delay Karl.
Sometimes it works, and sometimes Karl just grinds them into the cobblestones.
CheerfullyInsane