any fish from the lfs i would be wary of carrying parasites or other diseases (ick, velvet, worm infestations, bacterial infections, viral infections, etc). this is why we should quarantine all new fish when we bring them home before adding them to the main tanks.
i'd set up a 10 gallon if you want to have a 'feeder fry nursery'. it can be relatively simple with a sponge filter and some plastic plants weighted down. you could leave the tank bare bottom for ease of cleaning, since you'll be feeding those fry really well to 'gut load' them for feeding to the bettas.