2" anything would be lost in a 50. Get it and enjoy it my perspective. What others think not going to be the deciding factor. When time comes to move on then move on which could be donation to another, trade at LFS or excuse to get a bigger tank yet there's this little booger that has grown with you.
We put life in a box that will never come close to that they came from therefore drawing a line based on moral self imposed values seems rather foolish considering we have no clue what that fish is actually feeling. It eats. It grows. It behaves apparently normal. What's the actual concern?
So, I’ve personally kept tangs in everything from a 40 gallon breeder to a 90 gallon corner tank, and typically would pair a bristle tooth with a zebrasoma for each tank. While I wouldn’t put a tang in anything less than a 40g, there are a lot of things to consider:
What is the size of the fish compared to the tank, a 2” yellow tang is useless in a 200 gallon tank, and a 6” yellow tang is over the top for a 40 gallon tank. I have also purchased all of my tangs small, and I’ve noticed lower aggression rates when you introduce them small. I also believe that a fish slows its growth pace when in a smaller tank. I feed heavy with aquarium temps at 77 degrees and the tangs haven’t grown much over the last few years, but always appear to have have full bellies.
Try and quarantine and/or introduce tangs in at the same time, or, if not, get a good in tank quarantine box and keep the new tang in the qt box for at least a week so the other tangs get used to it. I have done this several times and have not lost a fish due to aggression. The alpha of the tank will still make sure to show dominance over the new fish when introduced, but had not killed a new introduction.
Size of the tank does matter, but swimming space and intricate aquascapes with a lot of swim through holes and places to explore are a lot more important in my opinion. The rockwork in my tanks usually have a lot to explore, and the fish are constantly nipping at rock and exploring the tank rather than just swimming back and forth.
Flow for tangs is a big deal, and a nice moderate flow keeps them active and healthy.
Understand that if your fish it grows the tank, it’s time for a new home. The reality is that if you kept a fish alive, healthy and happy in its youth, you stand a higher probability of keeping the adult fish happy and healthy. When you turn the fish into the lfs, it’s now up for grabs to anyone to buy it, and there is always the chance someone will put it into a smaller tank or same size tank as you.
At the end of the day, fish is a pet, no different than a dog or cat. Many of the fish we keep can outlive a dog or cat if kept healthy, some fish can live over 30 years. The second you purchase it, it’s now your job to keep it healthy and happy.