I am currently doing alot of research on the freshwater puffers because I would like to get some myself. The current (December 2002) issue of AFM (Aquarium Fish Magazine) has a great article on "Captive Care Tips for Freshwater Puffers".
, Puffers are quite aggressive and don't easily fit into community tanks. (Unless their tank mates are somewhat agressive, slightly larger and considerably faster). Puffers in general are very territorial so only one male unless the tank has plenty of space and hiding areas.
I do know that they love to eat snails and shrimp, I don't know how they feel about Red Crabs
Dwarf Indian Puffers (Tetraodon Travancoricus) only grow to about 1 inch.
Figure Eight Puffers (Tetraodon Biocellatus) only grow to about 3 inches.
Most others will get too big for your 5 gallon tank.
From what I have found on the internet, only the Dwarf Indian Puffer is purely freshwater, the other "freshwater puffers" actually do better in lightly brackish to full brackish water.
These sites have tons of great information about puffers.
You will also find an amazing amount of information if you do a GOOGLE search on FRESHWATER PUFFERS
If it was my 5g tank, I would get Dwarf Indian Puffers. (1 breeding pair or 1 male and 2 females maximum)
Personally, I am considering 4 Dwarf Indian Puffers (1 male and 3 females) in a 30g. I have 9 (1/2 inch) Tiger Barbs which I will introduce to the tank first and let them get established. Once they are 1 1/2 - 2 inches in size then I will introduce the Dwarf Puffers.
I hope this helps.