dwarf puffers are freshwater fish, so no salt needed. Not kept them, though I looked into it, but I know they are aggresive little fish and are generally kept in species tanks (as in, all dwarf puffers) though I think there may be a few potential tankmates. Betta is definate no though. I'm pretty sure the DP would make short work of the betta's fins.
You may have more luck getting info in the oddballs page.
Here's a link to get you started http/www.fishforums.net/index.php?showtopic=136616