Most angels need bigger tanks. Dwarf angels even. The minimum i've seen with ALL angels is 30g. Another thing to worry about if you put it in a smaller tank is that it has a higher chance of getting stress-induced illnesses. And then you have to worry about quarantining the fish, fixing him up.. and then wanting to put it back into the same tank where he got the illness in the first place. As for the whole coral thing, i dont know why you would even go the route of being "cautious" with a fish that is KNOWN to eat corals. What happens when it starts eating the corals.. do you return it? Or do you let it eat the corals and then have your tank become FOWLR
. The decisions are delicate and in your best interest to choose one before the other, instead of trying to have both. My suggestion is, keep the angels in a 30g tank MINUMA, and keep them away from corals. Corals are expensive and theres no need to risk them being eaten.