I have 3 yellow tangs, 1purple tang, 1naso, and 1 hippo in a 150.
How do I do it.
Keep yor fish well feed to reduce competition among rival species, namely in this case the purple and yellow tangs. Hungry fish tend be agressive since their body is not busy digesting food. How active are you after a nice big meal right?
Also I can't stress the importnace of shelter enough. Keep lots rocks so that the fish are not always in contact, also the rock allows smaller specimens to hide when being chased and prevents the agressor from following them.
Also tank length is helpful. A 150gallon cube say hypotically 4ft in length would not be very nice for tangs despite the same water volume when compared to a 6ft of the some volume.
As previously mentioned add them all at the same time so no one fish an establish itself as the dominant fish.
Finally when you have this high of a bio load in the tank you better have good filter system, which I think I have got.