I would recommend not using goldfish as they are high in saturated fats.
Cool water, cold water, marine and live bearers are all pretty well balanced as a feeder fish.
One of the notes to keep in mind is that many stores keep a copper solution in the tank to cut down on disease and there is some anecdotal evidence in Pelochelys bibroni of copper toxicity from feeder fish.
I breed my own guppies for my axolotl and he loves them. I put about 2 in his tank once a month just as an occasional treat. They are from very healthy guppies and I am sure they are clean. Breeding guppies isn't very hard so if you really want to feed them fish this would be a good idea (don't trust pet stores!!)
I just got an Oregon newt and they told me to get guppies to feed him. He died twenty hours later. Could this have been an answer to why he died? I had always fed pre-made food in the past but thought I would try this way because they said so. Guess I should have looked here first.
If the newt died in 20 hours, it was not the fault of the feeder fish (and not your fault either). A healthy newt can live for weeks without eating, and can survive all kinds of poor treatment for a long time. You got a newt that was so badly mistreated that it didn't have much life left by the time it got to you. Unless there was something toxic in your tank, that's the best explanation for why it died so quickly.