Gobies are excellent to watch. Their frequent border disputes and arguments over the best pot are hilarious. They make a great addition to any brackish setup. The true bumblebee goby will also be accepted into a community setup.
You can keep bumbleebee gobys in fresh water, however they wont thrive nor last as long as they would in brackish.
The only thing you can keep with DP's are ottos IMO.
well, you could put a coldwater goldfish into a tropical brackish tank and it would live - but would that mean it would be happy? do you really want to put a fish in with your puffer when it wouldn't live very long, may be bullied and would suffer?
I would stick to otocinclus if you must have a tank mate in with a dp. Cories can't move fast enough to get out of their way and imho are sitting targets for being tasted all the time.