Puffers are actually happy by themselves a lot of times they don't get along with other tank mates sometimes not even other puffers. But if you insist on adding something I would certainly not put a piranha in there the puffer will lose that battle.
One things for sure though - seriously research your fish or at least google them for at least 5 mins and you will quickly see that in no shape or form should bettas be mixed with puffers.
Equally balas and piranahas are ridiculous ideas for a 16 gallon tank.