Having more light penetration is the best for the zoas. I can't say much about the 30g since I had my tank custom built 24x14x17, close to the 20H but wider I think. I have a 250w MH and it works great.

My tank is acrylic and I haven't had a problem with scratching so far. Just use the scrapers for the acrylic and watch out for sand on it and you should be good.