Yes, you can use them as feeder fish. In terms of nutrition and safety, home-bred livebearers are the ideal. Gut-load them first, and obviously keep the feeders free of diseases such as whitespot and fungus.
But if you're asking "is using feeder fish a good idea", well that's a little more complex. On the whole, there's rarely any advantage to using feeder fish unless you're keeping very specific sorts of fish that won't accept anything else. Most predatory fish will receive a healthier, more diverse diet if given dried, invertebrate, and frozen foods.
The fish that should never be used as feeders are minnows/carps including goldfish and rosy-reds; they're too fatty for most fish when used regularly, plus they are rich in thiaminase. Even without these factors, the majority of commercial feeder goldfish and minnows are raised in abominable conditions. Likewise, even things like neons should be avoided because of the risk of NTD, which can infect a diversity of different fish, not just neons.
Cheers, Neale