The dwarves sold most commonly are freshwater. They will tolerate salt about the same way a neon does--okay for the short term, not good in the long run. Puffers are discussed in the brackish forum here on AC simply to keep puffer talk in the same location.
10 is okay for one or two. I wouldn't plan on keeping many tank mates with them, as they can be very nippy, and it's hit or miss. I had some in with a variety of fish, and they ignored the bristlenose, harassed the rainbow shark, chewed up the cories. They do best with a variety of crunchy foods, though this is not as critical with dwarves as it is with the larger puffers. A visually complex tank is crucial! Lots of plants and hiding places.