What kind of puffer? Sounds like a great addition to your tank...they always are
Puffers do well in captivity. They do get large but are not the most active swimmers even in the ocean and two seem to love human interaction although most of it is just a food dance
Im not saying to cram a porc in a 30G but please don't feel bad about your puff in a 260G
This hobby is nothing if not selfish and you have to get over that to a point. Feeling guilty is good though IMO and if more people did, you would see less fish in unsubstantial aquariums. Most of the people on here or most online forums do make the right choices though and research. I am talking more about the millions of people who do not join forums or care to make sure a certain fish will be ok in their tank.
It is a dilemma with that huge tang. He has to go somewhere and there just are not too many huge tanks out there. He should never have been collected in the first place. IMO there are many, many species collected that never should be. Why are Starry Puffers collected? They get 4'...FOUR FEET!
I think fish should be collected based on adult sizes they will reach and not the 3" they are at the time. So many people buy fish thinking well I have a few years before it outgrows my tank...I have. Its wrong though because then what? I can tell you first hand, its not easy to find a home for a 16" fish. I have tried in the past and eventually had to take him back to a store and feel guilty ever since. I have no idea what happened to that guy.
Good luck and it sounds like you will make the right choice.