Lots of questions! I do have a question for you about the kitty with the scabs--are they all over his body or just in one area? Are the cats all on a flea preventative? Any hairloss or raw areas of the skin? How long have the scabs been present?
It can be very difficult to get a cat that has always eaten dry food to show any interest in canned food. So while I like cats to eat some canned food, it may just be easier to not push that issue at this time. So you may want to just concentrate on transitioning from the Purina dry food to the Blue Buffalo dry food first, then transitioning to meal feeding, then go from there.
I also have 4 cats, with one of them being a bit pudgy and diabetic and one being a petite little girl. So I understand your issues! Each of my cats has their own food bowl and they all eat in slightly different spots. Three of them are on different levels of a cat tree and the diabetic one is on the floor apart from the other cats. Admittedly the petite one doesn't eat all her food and then the other cats scarf it down later. And the diabetic one nibbles on any food my dog leaves in his bowl. So it is a never-ending battle!
An average, overweight cat that should weigh about 10 pounds, should get about 165-180 calories per day. So figure out how many calories in a cup of Blue Buffalo and divide that into 180 (so if BB has 400 calories/cup, then each cat should get less than 1/2 cup per day). I would look for a reduced calorie food--I've seen cats do well on Chicken Soup light and then feed all 4 cats the same amount. Divide the total amount into 2-3 feedings per day--they are more likely to finish when there is a smaller amount at each feeding. Plus it "revs up" the metabolism each time they have a meal. You mentioned that they are only eating about 1/2 of a serving, but do verify that serving size based on caloric need. Bags often put down a higher amount of food than is actually needed.
As far as the scabs, they could be due to a food allergy. In which case you want to choose a food with completely different ingredients than what is found in the Purina foods you have fed. Any protein or carb source can cause an allergy, so a grain-free food may or may not be helpful. Maybe try a venison diet. The scabs could also be due to fleas, especially if the scabs are on the lower back and/or around the face and neck. If the scabs are mainly in the area in front of the ears, then a food allergy is quite likely. I would expect to see improvement with a diet change in 3-4 weeks.
I wouldn't add any coconut oil to the food--just added calories at this point and no evidence that it helps with weight management. There is a new Science Diet food out that looks very promising for getting weight off cats (and dogs) and helping them maintain a normal weight. Not a limited or novel ingredient diet, but another one to consider.
How that helps!
The cat has scabs on her back near the tail and sometimes on her face but for the most part on her back which seems to be ongoing. The cat is my girlfriends and she seems to think its anxiety related.
she has brought the cat to the vet numerous time and she says, "the vet gives her a steroid shot and its pretty much a bandage for a few months."
no the cats are not on flea preventative care. again girlfriend says, "Its not fleas, i comb her and i dont see any eggs or fleas"
there is some hair loss in the area of the scabs.
even though i plan to try the frontline product...what are the chances its a food allergy?