Kitties need to eat and eat a lot! They are growing. They should have multiple meals a day. Any change in diet should be gradual to avoid vomiting and other issues with constipation or diarrhea. As for the amount of food -- there should be general guidelines on the package. As for using different brands of food, that's fine as long as your kitty enjoys both brands. You always want to find food your kitty will eat rather than sniff and walk away from. It may seem that your baby has a ravenous appetite, but unless she is throwing up after eating, she's doing fine.
Oh -- and I could survive on chocolate and Cheetos -- every meal, every day!