If you're going to do that, you need to use the console, and clicking on the guard will display the editor id (of the guard), so you won't have a problem there. That also makes them the "current reference" so you can just use AddItem and RemoveItem directly without typing theirs in. However, you will need to know the Base Ids of whatever you want to give them (and remember to type in a quantity - I keep forgetting that bit).
You don't want to change their equipment in the CS, because some of them are "generic guard" and their colleagues back in the city will also be changed. Just do it before the battle in-game and only the current instance is affected.