I would hold of on the tangs, they more swimming room. Chromis are nice and hardy fish to start of with. As far as corals, it seems like your tank is equipped to keep soft corals, mushroom, and a couple of hard corals. I think I would go with an extra inch for the sand bed so you could have more room and less oxygen for denitryfiying bacteria to grow. Its good that you are doing your research. GOod luck with your tank.