Rather than going through all this effort with breeding your own guppies in a kiddie pool yada yada, why not go with a different fish? Do puffer fish count as predatory for you? You already said you knew how to grow snails, and you could grow those in the 10 gallon with your brother's dojo (I REALLY hope that dojo is moving to a bigger tank soon since they need 30-50 gallons and friends, but that's a whole 'nother ball of wax). All of the larger puffers are brackish however, but if you're willing to do a brackish tank then there's a wide variety of puffers that would be happy in a 55 gallon tank. If you go with a Figure 8 puffer instead of a Green Spotted you might even be able to have tankmates like a dragon goby, bumblebee gobies, or mollies with it, depending on the puffer's personality, or instead you could do a group of South American Puffers (they'll need teeth trimming though). Puffers tend to be very personable and active, so the tank will seem less boring even if you don't have many fish in there. If you have to have a freshwater tank, Dwarf Puffers are the only pure freshwater puffer options, which are likely way too small for your liking. South American Puffers only require 1.005 salinity, which is pretty "light" salinity as far as brackish tanks go.
Up to you, but I'd say this would take far less space and be easier to feed than trying to go with piscavores instead of the invertebrate eating predators like puffers.