Well, imo (humble at that) it is okay, but let me explain why. First, its not fair to keep a fish who naturally eats a certain food, then make it exist on flakes.
(eew) Second what is the difference between the many of us who have livebearers and don't use a seperate tank or breeder net? Alot get eaten, but really what were we going to do with 50 kazillion fry anyways?
Also, a fozen fish is a dead fish, any way you look at it., It had to be killed somehow, and from what I hear freezing is one of the more cruel ways to go. (and not acceptable euthenasia) Also, why not save some $$ on fish and treatment, if you are feeding them live anyways? Now it isn't for everyone obviously, but my bettas enjoy a good fry now and then. 
