It's possible to keep nearly anything in a 55g, as a baby. That doesn't mean someone who only has a 55gal should buy any fish they want. It's bad practice for someone to buy a fish they won't be able to keep for its whole life, or to buy a fish for a tank they're planning to eventually get. Too often, life gets in the way and the fish suffers.I understand what you are saying. I also did not clarify that the YT is destined for the 180 eventually. My issue is with "you can't" I despise those words!! I am the person that if you tell me I can't do something, I will immediately go out of my way to prove you wrong.
It is absolutely possible to keep a Yellow Tang in a 55. just be aware it WILL outgrow it and need to be rehomed.
Redtail catfish are a freshwater fish that can get to 4-5 feet long. I can get a baby at a few inches long and put it in a 55gal freshwater tank, and it'll be fine in there, until it grows. That doesn't mean I should buy a redtail catfish, if I have a 55gal tank and no plans to upgrade.
The phrase "a redtail catfish can live in a 55gal" is misleading; it sounds like the fish can be kept in the tank for good. Same for the phrase "a yellow tang can be kept in a 55gal". A baby yellow tang, or a baby of any reef fish, can be kept in a 55gal. Until it grows. Then it can't be.
When recommending fish for someone, it's best to only recommend fish that can live in the tank size given for the fish's entire lifespan. If something can only live in the tank as a baby, you really need to clarify that, and it's still not an ideal recommendation. There are plenty of nice fish without needing to get something that's going to grow too big.