I have kept many, many tangs in tanks together with no aggression. If you feed them enough that they grow 1-2" a year and have hiding places where they can completely get away, I have never found them to be all that difficult. I mean a lot of food - probably enough that they could make your N and P rise. When I say hiding, I don't mean the shelves and caves that reefers like, but stacked rock with actual security.
When I had the big tank set up, I would get aggressive tangs all the time from people who did not want them anymore, but they just ended up being part of the group when I got them - most of them were skinny and starved although their previous owners swear that they fed "all the time." I am talking about enough food that the foxface that you have would be too large for your tank in 3-4 years at 8-9" long and an inch thick.
About 50% of diet from NLS pellets, 30% from mysis and 20 from other stuff like nori and other frozen treats.
I have found that "introduce them at the same time" or "stocking order" or "same shape" to be completely random although I am sure that some of these have worked for some people like all random things do. Food, security and room to swim are the keys, IMO... and people fail at most of those.