I would breed N. Pulchers/Brichardi. They can produce large numbers and they don't eat their fry! Once they have one spawn, they will spawn again, and the first spawn helps care for the first. Then, when they are an inch or so, pull them out, feeding time. And the parents, which you only need two or three to form a breeding pair/trio, will keep on laying eggs, producing fry. I do Neolamprologus Pulcher "daffodils" myself.
Is this for feeding one or two fishes? If it is more than that you have to be looking into something else. I go to the store and I buy pounds of fish to feed my p's.
convicts breed very fast mine when i got them breed within a week and a half and they had baby which hatched 5 days later within a week after that they are able to be feeders they have alot of babys 20-30+
A group of 1-2 male and 4-5 female Saratherodon mossambicus (Tilapia) - gold or regular - will provide tons of fry. You could maintain a group of 4-5" fish in a 55g. You'll nearly always have a couple of females holding...and they have lots of babies (up to a couple hundred).
They're mouthbreeders, so the fry come off of holding females at a decent size.
Trying to breed and grow out guppies or other livebearers at a rate fast enough to feed other fish won't work unless you've got A LOT of fish breeding.
Convicts would be an alternative but you're limited to the output of one pair at a time in a given tank.