Well, FWIW, here is my take on the cat food situation. I’ve been breeding pedigreed cats for 7 years, plus worked as a vet asst. for 3 years. I am FAR from being an expert on feline nutrition, but at least have spent years arguing about it with other breeders!
Re: the kitten food/cat food problem. What brand of food are you feeding? If it is Iams, Science Diet, Eukanuba, or one of the other premium brands of food, you can raise a kitten on it from weaning time. I don’t have the links handy at this time, but at PubMed I found several abstracts of articles saying that most kitten food actually had far more calcium and other nutrients than kittens needed; if you read the labels of many cat foods carefully, you will often see that they say “Complete diet for cats and kittens.” I’ve raised many kittens on Iams or Eukanuba adult food and they all did splendidly.
However, if you are feeding a standard food like Purina Cat Chow or Whiskas, you might want to at least mix it with Kitten Chow for the extra nutrition and easy digestibility. You can feed straight Kitten Chow to both the adult and the kitten, but you will need to keep an eye on the adult cat’s weight.
I highly recommend the premium foods (personal preference Iams and Eukanuba). Although they appear to be much more expensive, if you will compare the recommended feeding amounts you will see that the cats need only about 1/2 to 1/4 premium food as they do the ‘regular’ food. That’s because the companies making premium foods have researched and refined their recipes to provide the highest digestibility with the most balanced nutrition. They also guarantee their ingredients - that is, they don’t shop around for the cheapest bid on, say, chicken meal - they use guaranteed quality ingredients from suppliers they trust, regardless of the current price. This is also one reason premium foods are so expensive!
(One reason this is important - several years ago a bunch of dogs died because a pet food company unknowingly used a grain product (corn? I can’t remember) that was infected with a deadly fungus.)
Anyway, judge a cat food by it’s ingredient list. Cats are obligate carnivores and have a very short digestive tract designed for digesting nearly pure meat-based protein. Ingredients that are hard to digest, like the cellulose in grains, is mostly filler that just goes in one end and out the other. (Another advantage to premium foods - less food intake + higher digestibility = less stool and less odor in the litter box.)
Most ‘standard’ cat foods will have corn as their first ingredient - do you think corn is really a suitable diet for an obligate carnivore? It’s been broken down so that cats can digest SOME of it as it passes through, and bunches of necessary nutrients have been added, but basically it is a vegetable diet modified to make it edible for cats.
A premium food will have some type of meat listed as the first ingredient, and often the second and third ingredients! This may just be “meat meal”, but at least it IS meat. Preferably it will be from a single named source (like chicken) rather than the generic ‘meat’, which can be pretty much anything. The first ingredient in Eukanuba cat food is simply ‘chicken’ - the natural meat content is so high, and so unaffected by their processing, that they don’t even have to add a taurine supplement to the mix. Needless to say, meat is more expensive than corn, hence the higher price.
Purina Kitten Chow used to be more of a ‘standard’ food, but they changed their recipe a couple of years ago (?) and the first ingredient listed is now chicken. It’s a much better product than it used to be, and I can feed it to my cats in a pinch (regular food not delivered) without causing an outbreak of diarrhea and vomiting.
Oh, and if you’re feeding one of those ‘store brand’ foods, like Kozy Kitten or Special Kitty - don’t. You might as well just grind up money and feed it to your cat, because you are wasting it, based on the level of nutrition per the amount of food eaten.
The older cat doesn’t have a weight problem-she was getting a little plump before we got the kitten, but she lost about a pound and a half in the first month of having a hyperactive playmate. Since the baby did eat mostly kitten food until she was about three and a half months old (when the older cat decided to give up defending her dish, and then found out that she liked kitten food after all), she should be just fine.