The numbers of fry you intend to keep as well as up to what size would be crucial in deciding the tank size..... (what are they going to be fed to)....
You could probably grow a single guppy drop to adult size in 15 or 20 gallons. That would not give you very much food for your predators though. It can easily take them over 2 months to get that size so it would be like one guppy for food every 2 or 3 days. If you are feeding a predator that needs to eat every day you may want to consider a 55 as the minimum to raise that one fish's food.