Diamonds are the most aggressive goby I've seen, and I'm sure others will attest to this as well. That being said, if you had them in the tank together for awhile and no aggression, one just may have died from natural causes. I've read in books that two different types of gobies can't be kept in the same tank, but then hear millions of people say they do it without a problem. It depends on how much food is available for them. IMO, two sand sifting gobies in a 55G might be too much, especially as they grow and require more food, so you may have had an aggression based death. I have two yellow watchman and they're fine together. They chase each other occasionally, but then sleep in the same cave, so who knows.
As to gobies and dragonets being together, absolutely CAN be done. I have two watchman and a mandarin with no problems. I've also had a Dragon Goby with a mandarin without an issue. Most of the time your sand sifters are not going to eat copepods, unless they're starving.