spotted puffer's do really well with a lil salt in their tank they are a brackish water fish no difference between fresh though I have been a big proponent of the use of spotted puffers as nasty lil guys for the small tank... I think most people will agree get a couple of spotted or a spotted and a figure 8 puf...
One of the drawf puffer species would be cool, but your best bet would be a school of exodons aka bucktooth tetras. Imagine tiger barbs on steroids that haven't eaten in days. That's what exodons are like 24/7.
For pure mean do the puffer,
for fish eating killers try 3-7 pike top minnows (Belonesox belizanus), they will stalk and munch mollies just dandy, they get 6-8" long and are live beares that breed almost as quickly as guppys. They like a little salt in the water.