Using baby guppies as live food and are they nutritional?

Yes it will be fine to feed them to your other fish. They should not be the only source of food but will be fine for an occasional meal or treat. Its not that feeder goldfish don't have any nutritional value it is simply that they should not be fed as an only food source. In most cases no one organism contains all the nutrients another organism needs. It is the same concept of people who raw feed their carnivorous pets (dogs, cats, ferrets etc) needing to switch the type of meat fed around, also really no different than you trying to live off only beef or only carrots for that matter. Its not that they lack nutritional value it is simply that they do not contain 100% of the nutrients you need in the long run, its fine for them to be part of the diet they just shouldn't be the only thing in the diet.