What you have got to remember when breeding feeder fish is you really want fish that are prolific breeders, Severums, convicts are good examples of this. They lay a lots of eggs on a regular basis and can produce many young. Guppies are all so fish that breed a lot, however, there is not much of a meal in a guppy. Also, remember that fry take a while before they are big enough to make a worthwhile meal so unless you've got several sets of parents breeding at different intervals, you are not going to have a steady supply of feeders. And finally, your tank isn't big enough for these type of fish anyway.
Your best bet would be to buy them and put them in quarantine for a week or two, this way you would at least have a steady supply of fish.