It is perfectly okay and recommended to give a vairied diet. I was suggesting that if you are planning to feed only one main food source to make husbandry easier, that I would recommend feeding nightcrawlers/earthworms as they are the most nutritious of the available food items. I would recommend not feeding fish and frogs as they normally carry a large parasite load. It sounds like you will be getting the captive bred juveniles that were exported, congratulations on getting the two!
I have kept many many species of frogs, especially less kept, rare or endangered species. I have helped zoos in different parts of the country maintain and breed various species of endangered amphibians over the years. I don't like to boast though, there is always someone out there who has way more knowledge than the next, and it is all about relaying the information to the next guy so we can keep these animals around on this Earth. I will be flying to Argentina on the 19th next month to research the natural habits of the frogs there, as well as doing some mission work. Hopefully it will be a successful educational experience that I can share with everyone!