If it's a late boil addition, it will be sterilized by the heat in a matter of minutes, so you're good to go. If it's a post boil addition, I would just wash it thoroughly in filtered water with a few shots of an organic fruit/vegetable spray before you add it. I've always just added things directly to the primary, no bags. Not sure about how much zest to use. For a smaller batch, I imagine the zest of one orange would be a good starting point. I tend to shoot for the lower range of what I see other people doing and then work my way up from there to my own taste preferences. The worst thing that can happen is that the orange overpowers everything else - in which case you might as well just be drinking a screwdriver.