Let me see if i can address the various items.
Firstly for Sku quantities, you can indeed create a 1 off sku and set its quantity to 1, and there's a radio or drop down that asks "do you want to track inventory?" and "If none left, disallow purchasing" (something like that). So you can have it either not show, or the default transformation of an item will have the "Buy" button disabled automatically if it's set up this way.
Not sure also why you would create a new category...may be good to have one off skus in a "one off" sku category for easy display (select all 'one off' skus), but otherwise i also fail to see what this would accomplish.