Never, ever cook chicken with bones. The bones turn hard and can puncture the inside of the cat. This can kill them!!!!! If you feed a cat something with bones inside, be sure to give it absolutely raw.
Kittens need to be able to eat throughout the whole day. The extra's you can give them regardless. They need to grow a lot so they need food. If they like to eat the adult food, it's oke. When I have kittens they have acces to dried food, kitten variety. After a while you notice the mum and all other cats in the house eat the kittenfood and the kitten is having the adult food.
So, with more cats in the house it's no problem to only put the adult food. The Maine Coon is a big cat and growing slowly. If one is staying behind in growth perhaps it will be bigger after a year. It differs from cat to cat, even when they are brother and sister. Always remember it will take them about 3-4 years to reach their final weight and hight.