Aquarium Co Op has their MBU puffer (Murphy) in a 300 gallon tank. They made a lot of videos about Murphy and they often say that the 300 gallon will not be his permeant home. I have never owned one but you can see the videos and decide what you think
IMO If your mbu puffer is still a juvenile, then that 260 gallon tank is plenty big enough for him. But mbu’s can get to 18-24 inches or more, so later on- couple of years, you may need to upgrade. I’ve had green spotted puffers & Amazon puffers, but not Mbu. You’ll love their personality, they are one of the few fish that can see us up to 30ft out of the tank. Feed him lots of snails, clams, shrimp, mussels- I just went to grocery store and bought fresh seafood, they love it. Or freeze dried krill. They will get along with other small fish as long as their well fed & see you as their person. Neither of mines ever ate mollies that were with them, they were too worried about what I was going to fed them. No long fin fish with them.