The man in the shop is correct IMO, puffers don't usually get on with anything, let alone other puffer species. If you have a large enough tank you may be ok, but on the safe side, don't mix them. The south american puffer is supposed to be one of the most peaceful, but I know from first hand experience that they will rip the fins from fish four times their own size.
Do you know the scientific name of your new puffer? I only ask as green spotted puffers are often sold as spotted puffers, but both require different water conditions, the spotted puffer is a true FW fish but a green spotted puffer is brackish, though can be kept in FW while young.
As wilder mentioned, did you cycle the tank before adding the puffer? read the pinned posts on cycling if you are not sure.
Jon