I have 4 pea puffers in a heavily planted 10 gallon species tank and have to say it is not big enough for them. I'm in the process of moving them to a 20 gallon when the cycle is done. These are aggressive and territorial little fish - don't let the size fool you! As juvies they tolerate each other well and trick you into thinking they can happily live in large size groups forever, but as soon as those hormones kick in forget it lol.
I feed mine a wide variety of frozen and live foods including frozen mysis shrimp, frozen bloodworms, frozen mosquito larvae, live or frozen daphnia and live brine on occasion. Their tank is stocked with tons of trumpet snails for them to snack on between feedings and I also raise ramshorn snails for them as well. Some people think pea puffers won't eat trumpet snails because of their trap door but that is totally not true. I'll also be adding a colony of cherry shrimp to the 20 gallon as I hear they eat the baby shrimp (and sometimes the adults!)