Understandable question.
I had a hippo tangs in my 30g zoa tank because it was a baby, 1 1/2" each. Kind of like how I have had 18 baby clowns in my 30QT tank for the last month. I don't believe in throwing babies in large tanks until they can fend for themselves. Where I would otherwise agree with you that you should not ever place fish in tanks that are rated to small for them, for me it is a graduation process. I already own a large tank they just were not ready for the open water. You should never buy a fish you can not accommodate in the long run. The zoa tank was well established, had been up for several years, and was a very healthy tank. Hippos babies are very sensitive and I wasn't about to drop them in my 240g until I was confident in their health so I put them in the 30g for about a month because they are fast growers. I came home one day to a cracked tank which was more than likely what triggered the slime ball to nuke the tank. All the fish except a clown were dead. The devastating blow was my newly paired madrins that I had just finally got to switch to frozen brine. It was crushing... One thing you all can learn from this is that hardy clowns along with a tank jammed full of zoas made it through this painful ordeal just fine.
I understand my incident was isolated but isolated incidents happen to everyone. Why compound it with a fickin grenade sitting in your tank waiting for the pin to be pulled.