I have to throw my 2 cents in here, on the whole feeding live fish to other fish. I don't like it, and no... it's not because you are killing the feeder fish, it's because:
1) Your typical feeder fish (goldfish) are not even a naurally occuring species and have round about no nutritional value therefore a really bad choice for fish food.. it would be kind of like you trying to stay healthy on spam.
2) Feeder fish are prone to disease. Horribly prone to disease. The most common and rampant is tail and fin rot, followed very closely by mouth fungus. Both of these eat throught the affected tissue, and sometimes the bone resulting in disfigurement and or death. Not exactly something you want to introduce into your tank. As for feeder guppies, see all of the above,(with the exceptions of being naturally occuring, they were at one point, and they have a slightly higher nutritional value) and then add rampant infections of almost every conceiveable kind.