Always going to vary in size and growth based on sex and dominance. As far as the food, as long as they are eating something your good. As they grow and get accustomed to the routine you can vary the diet by slowly changing it up. Maybe once a week give them some mysis shrimp or dried bloodworms or something. If they don’t eat it don’t feed them again that night/day, just go back to the flakes the next day.