The minimum tank size we recommend for P. hepatus is an 8 foot tank; they do, after all, reach one foot in length rather quickly. Obviously, a ten foot tank would be even better. P. hepatus, can be on the mean side once they are full grown, especially towards fish that occupy the same ecological niche. (I have never tried to pair them so I cannot speak to that particular option). So, that means I would avoid the tang groups which tend to be mean (such as zebrasoma tangs) but if you go with the ten foot tank, that does open the door to various species of naso tangs which are my favorite in the tang group.
While those who follow this thread know, I do not recommend occupants for a variety of reasons, I am always happy to analyze any potential stocking list for compatibility with themselves and the environment that you provide. My personal preference is to have many small fish with interesting behaviors as opposed to large fish which make whatever tank you have appear small.