yup, I started doing this after being home tinkering with Fleshys tanks. I have a heavy duty airstone in my overflow right now. In previous tanks I have used it to wipe out Cyano with good effectiveness (combined with jacked up flow)
no, run it all the time, or run it opposite the lights. You are trying to combat the "off light" passive release of co2 by your macro algaes. Most important at night, but for ease, I would just run it all the time.