Ok, ReeferDude, maybe I won't get flamed for this reply cuz it's too long .There are many variables to consider when mixing tangs, including type, timing, sizes, and age. I think the most important one is the selection of the individual specimens, and their age,(juveniles work out best) but you'll want to increase your odds in every way you can before giving it a try. And you need a back up plan if it doesn't work out because there is a better than even chance that it won't. It's not likely, but if they start tearing each other apart instead of just posturing, then it is not going to work out. They will need to be seperated immediately and you will have to know in advance which one stays, and where the other one is going. If you only observe fanning and posturing, without injury, there is a good chance that one fish will accept #2 status, and #1 will let him. Your tank is big enough but I wouldn't introduce them there. It will be too difficult to seperate them if neccessary. All fish should be quarantined but especially these two. I would assume ich will show up in the beginning, even if they get along ok. I have had success acclimating and observing fish in seperate tanks, and weeks later adding the more aggressive one to the less aggressive one's tank when I thought the time was right. I wouldn't put these two fish in your tank until they had been peacefully coexisting, and disease free for at least a month. They are each the most aggressive species in their respective genus' and are fairly aggressive overall toward other community fish, so establish those others you wish to keep, long before adding these two. Of the two I would say the powder blue is generally more aggressive but most purples have a mean streak too, especially toward any new fish added after it is established. Individuals will surprise you though. I am amazed by how much my sohal defers to the considerably smaller and shier naso. They are usually swimming together in formation like best friends. When the sohal occasionally nips at other fish, the naso will get between them and break it up. This is unnatural behavior but captivity brings that out. I observed the relative passivity of the sohal in the dealer's tank over several weeks. I decided to take a chance, knowing that I had another tank for him if it didn't work out. I have been transferring fish, corals and rock between four tanks, over the past year, in anticipation of tearing them all down and stocking a new 180g over several days next fall. I have been very patient and it has paid off. Best luck, HTH.