Hot water/vinegar should be okay. Most forms of parasites and bacteria can't survive for long without a host anyway.
I got a reply from OFL and she says that since different species are involved, she thinks it might be external parasites in the tank. Based on that, I would suggest you try a product like Hikari HealthAid PraziPro or General Cure.
Here's her exact reply:
While anything is possible......and especially if the fish are related and came from the same fish room....possible it is a tumor issue, however, more probable especially since it is different species involved would be external parasites of some type...since all the fish are symptomatic....its environment related or a new fish added that was not QT that was infected-infecting the tank would be my best guess....