It means by asking the question you already know the answer to it. Sort of like me asking do these pants make my butt look big. If you have to ask...
180 gallons sounds big, right? It does. So does my 210 gallon. However, that is just the gallons rough estimate. You now need to factor in things like rock, substrate, and things like power heads that take up space. Now that 180 gallons is a lot less. I haven't mention corals yet because you may start with frags. Frags turn into colonies.
A lot less means less space to swim. Less space for quiet time. Less space for sleeping or that right spot to settle down. Less space means possibly more aggression because of less space to retreat. Less space means less natural feeding grounds for fish the forage.
Lastly, less space because you mention fish that can reach 12" in size. Not one but two or more. And while yes, they may not reach that size in captivity, it does raise the question just because you can, should you?
So when the person you replied to said you already know the answer, you sort of do. This isn't about tang police or anything else other than trying to put it into perspective that the tank size empty does not equal usable space for the number of fish you want. Furthermore there really isn't a gallon per fish rule in marine reef tank system.
Lastly - the 600 gallon comment. You can stock these fish in 360 and up tanks. There are members here with 400+ gallon systems with a boat load of fish. More tangs than you listed not to mention 60+ damsels. You can do it. However, their system is designed with it from the start. That means tank, aquascape, sump, etc.
I wont' get into yes, no - was just trying to probably state the obvious
