Looks like H. Acuminata to me as well. I've had mine for 6 months with no issues. I bought him small as well. If you do, make sure you qt as they are very prone to parasites.
I agree those look like accuminata. The Hawaiin species is not easy to find. I've only found it once and some one else beat me to them. They do get quite large....especially in the top to bottom dimension.
They are not reef safe Heniochus. The reef safe ones have the last black bar before the tail fin ends in a point. The best way to know for sure is the price.... Non reef safe Heniochus are fairly cheap while reef safe ones are much more expensive usually in the $100 range and rare. Very few stores sell them unless it's a special order