I agree with you that the plates of dry food should not always be full. See below for a feeding schedule and how much to feed. I also agree that your older cat Tincy's vomiting should be a concern, and her constant eating, especially when she's not hungry. First a disclaimer: I am not a vet. My "cat knowledge" such as it is was obtained by researching various issues over the past several months since getting my first indoor cat. One of the things the vet asked at my cat's examination was, "Any vomiting?" So yes, I think vomiting should be a concern, possibly enough so to see a vet.
It is my experience (especially when I had a dog) that vets also know what, how much, and how often animals should be fed. Generally, vets also have training tips regarding behaviour, such as how to keep one cat from stealing another cat's food. Cats really do need their canned food, I am told, because they tend not to drink enough water to keep properly hydrated, which easily leads to urinal tract health issues. I understand this comes from being desert creatures by nature who get their fluid from eating live prey. But indoor cats on dry food are not getting fluids from prey, dead or alive. Hence the need for some wet food from tins. They also need access to fresh water at all times.
How Much to Feed
Use the guidelines on the package to determine how much food your cats need. For dry food, I feed Royal Canin, which guidelines are based on:
- body weight
- level of activity
If your cats have been "grazing," i.e. eating all they want from full plates all day every day long term, they may be obese and in need of a special diet as determined by the vet. That aside, levels of activity include things like:
- walking from bed to feeding area, litter box, and your lap (low)
- the above plus running and playing an hour in the house (normal)
- all of the above plus significant additional exercise (high)
I walk my cat on a leash around the block and let him climb trees in the backyard. No way is he satisfied with "low" or even "normal" rations. He needs to be fed for "high activity" levels. Please note: Those "levels of activity" are my own calculations derived from a combination of the following:
- a reference in my readings about obese cats not even having to walk to their food because it is brought to them on plates vs in the wild where cats have to stalk and hunt down their food.
- my own cat's need for "high activity" level rations.
I got my "feeding schedule" from Jackson Galaxy, who describes himself as having worked with cats for 25 years in shelters and on TV shows. Admittedly, my reason for seeking advice on the internet was different from yours; it was in the middle of yet another night that my cat didn't let me sleep because cats are night prowlers. Feeding schedule, Jackson Galaxy says in Training Your Cat To Let You Sleep, is crucial to modifying a cat's sleep-wake rhythm, as well as other behaviours. My point: He provides instructions on feeding schedules.
He explains a cat's psychology around eating and how its digestive system works, which suggests that a regular feeding schedule is a good idea for all cats. He recommends six to eight hours between meals. Watching the video will help you figure out the best feeding schedule for your house. He also references a previous video on feeding that you may be able to find.
For more tips and ideas re feeding schedules, see also How many times a day should I feed my cats?