Hi there. Long time guppy breeder here. You can basically crush any flake into powder (or almost powder) like state and feed them that. Guppies are not very picky eaters. For the first couple weeks, I tend to feed them 3 times a week at a very little amount and since I have my fry in a 5 gal, I do 30% water change every week. As for selling them, you can probably start selling them at around 1-1.5cm. Some will start showing color by then, depending on strain. my red snakeskins usually start showing at a bigger size than my tuxedos.
Justy FYI, once they start they won't stop breeding lol. I find 5-15 fry in my mating tank literally every week (i'm sure more but some of them get eaten if they aren't smart enough to hide in the plants).