Tangs are distance swimmers. They like to go as far as they can without turning. Without that long stretch they get cramped in and stressed. For fish that are already easily stressed, you start setting yourself up for failure via ich.
To tag on with the dwarf angels, I have a coral beauty in my 55 reef. Hasn't really hurt my coral, from softies to sps
, but he has nipped at them. In hindsight, I'd ditch him in a minute for something else.
In terms of puffers, the only one I'm aware of that is closest to being reef safe is the blue spotted puffer. Beautiful puffer, but loves to eat clams and would prob make a meal if any inverts in the tank too. I'd have one in my reef, as I don't house crabs/shrimp with my marine betta, but having clams is more worth while for me.