If you don't eat you won't produce waste very long either. Meanwhile your fish do need to eat. All of the fish need food, not waste, to survive. With the neons you were probably feeding more than the neons would eat so you got some of the food to the cories. Getting food to the cories without a food excess on the tank bottom may be a bit more of a challenge with the guppies but it must be done or you need to return the cories to the store and give them a chance.