I wouldnt suggest more than one type of puffer in a tank becuase some are more agressive than others which means the docile ones will be picked on a great a deal.
What kind of FW puffers are you talking about? Dwarfs and South Americans?
forgot about congos but DP and SAs are the common FW puffers.
I know that a member here has done it with some sucess but it does depend on both the puffers personality(s) and the decore/tank size. Lines of sight should always be broken up to keep down on agression as well as caves/hidey holes provided as escapes and sleeping areas. In a larger tank there is more room to escape but i still wouldnt suggest putting two species of puffer together.
If you're talking about any other puffer species than dwarfs and SA's the chances are they are too large to fit in a 20g (ie an Mbu or Fahaka) or they will need brackish conditions to marine as they age (ie Green spoted puffers , Figure eightsare BW). Some LFS sell these as FW when they really are BW.