DPs, IME, have a higher rate of having intestinal parasites than any other fish, puffers included, I have ever had. I would recommend treating them as soon as you get them. Just remember that treating the tank doesn't do any good, you have to treat their food.
Yes, they can often successfully be kept in larger groups. The more you have, the higher chance of problems, so that's something to keep in mind. There is someone I know that has a huge herd of them in a 92g tank I believe. Their behavior is different in larger groups, they seem to be more into loose schools than when they are in smaller groups.
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Puffers:
Auriglobus silus x2, Tetraodon travancoricus x1, Tetraodon miurus x1, Tetraodon turgidus x1, Tetraodon nigroviridis x2, Tetraodon baileyi x2, Tetraodon lineatus x1, Tetraodon palembangensis x1
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