The only puffer you could keep in a tank that small is a dwarf puffer. They live in pure freshwater, so salt is not a problem. They are extremely aggressive, so only they can be kept with each other in a tank that small. They have specific diets of live and frozen food which must be fed regularly, in rare cases will they eat flake, but even then it is not enough to sustain them. Snail are also eaten readily and will help keep their teeth down.
In a 5 gallon tank you could keep 2 DP's, and in a 10 you could keep either 3 or 1 South American puffer, which will grow significantly bigger but is also more peacefully, so you may have better luck tank mate wise. The SA puffer is also freshwater and has similar requirements, except they are known for their exceptionally fast growing teeth, and eventually you may need to trim them.
Puffers are extremely messy fish, and you should have a filter that can roughly recycle the water volume 10 times in an hour (IE for a ten gallon tank you would need a 100gph filter).
Also make sure to have lots of cover and plants to break the line of sight between puffers and other fish, as this will lessen aggression.
Read some of the articles in the Oddball forum for more info

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