I have fed fruit flies to FBT's before. They are happy to eat them as they will try to eat nearly anything. They are to small for a dietary staple though. I wouldn't worry to much about introducing parasites with wc bugs either. Many people including myself have fed wc bugs from time to time for years. I have never heard a first hand account of it causing a real health problem. As long as you stay away from dangerous looking insects(bright warning colors, stingers, pinchers, etc.) your frogs shouldn't have a problem.