Fantastic pics and video - glad that they've settled in okay (a couple of times I've gone to give them their food and they're not there any more, which has been weird).
DPs, when given enough space, are one of the few types of puffers who can often get along. It depends on the individual. Some will, some won't.
I wouldn't recommend more than 6 in a 30g, not with all those other fish in there. They may be small but they are territorial and have a pretty heavy bioload for such tiny fish.