What's the best food for multiple cats of different ages and health?

The best food with the most research are typically science diet, Purina or royal canin. You can find any formula that is made for young, adult cats to share. They make an optimal care or indoor adult made for young adults

Congrats on your new kitties! Given that you have a variety of cats, I would get a high quality cat food as opposed to brands for specific breeds. While I do feel that foods for breeds are worthwhile, it's hard to make sure each cat eats their brand specific food when you have a variety of cats. I feel the food for brands are worthwhile because I do find they tailor to the breed's specific need. Royal Canin Persian, for example, have a specially shaped kibble that makes it easier for Persian cats to pick up with their mouth. Persian cats can have a different shaped mouth due to the shortened nasal passages, and I have found my own Persian had trouble picking up cat food. The Royal Canin Persian was very easy for my cat to eat, and I feed him that. In terms of brands, I recommend Royal Canin kitten food. Other good brands include Hill's Science diet, Purina ProPlan, and Iams.

There is no reason why the younger cats can not have this food. Young adult (1-6) is suitable for all adult cats but annual monitoring of urine and body condition in cats over 6 is recommended to monitor kidney function, changes in which may necessitate changes to a mature/senior range

Science diet and Purina have very good cat formulas for cats. I would find one that is appropriate for your cats age and health status. For example, they make a hairball formula if that is problem or an over weight formula if weight loss is needed there is no benefit of feeding dry over canned, so it is personal preference for that.

Any major brand name that has an indoor care cat food line would be good. So this would mean something like Purina indoor cat, pedigree, Hills or Royal Canin. If you want to change the food do so slowly as sudden food changes can cause diarrhea and inappetence in a cat. So gradually mix the new food in with their old food, increasing the amount of new food over about a week time.

Rather than leaving dry food down I would feed each cat wet (canned ) food twice a day (kitten food for Chia and adult cat food for the other cat) and Chia can be fed a third time late at night for a third time. That way you ensure that each cat gets the food meant for it and the amount necessary. I recommend wet food over dry food as it has less carbohydrates and extra moisture. Cats are obligatory carnivores and should rather eat animal protein than carbohydrates.

Yes, I personally like Royal Canin diets. For her age, just the regular Adult food is fine. ( https://www.royalcanin.com/ca/en_ca/cats/products/vet-products?specialneeds=0ad835cc921d4d0cb3ee61a8cc461fbe&feedingperiod=78.3&filter=643ceeb4-c516-477f-96db-fd7ee1b41ec0 ) She can eat either wet or dry; whichever she prefers and what you can afford. Canned food can be very expensive if that is all you are feeding. You can also feed a combination of both wet and dry if you'd like. Feed her according to the directions on the packaging. If you do both wet and dry you need to make sure you don't overfeed her. She is already at her maximum weight so if you notice weight gain you will need to back off on how much you are feeding.