The information they gave you is terrible. I kept dwarf puffers for a number of years, and while some of them may survive in a tank as described, they would in no way thrive.
1. these guys are little, but they are intelligent, active and territorial. that many in that small a space WILL end badly.
2. I suppose they can live in a bare-bottom aquarium, but it will not be optimal.
3. They don't "require" a planted tank, but they do need something. these guys are intelligent and curious and will spend a lot of time exploring the tank. Having something that is heavily planted/decorated with lots of crevices and things to check out will help to keep them entertained and -can- help to keep their blood-lust in check.
I kept 5 in a 29 gallon tank that was pretty heavily planted, and there were still some aggression issues from time to time. If you really want to see their personality and natural behaviors come out, give them lots of space and "stuff to do". They are fantastically entertaining little guys.