Mia sounds like what I call a "cat's cat". She prefers the company of other cats to people. I had one like her at one time when I was breeding. Jazzy came from a breeder that really didn't spend much time with her other than to feed and clean their litter trays, so she never got a lot of handling. She didn't like being picked up, didn't crave attention, didn't enjoy being stroked, but really just wanted to be with the other cats. They were company enough for her and she was content.
As far as Mia nursing on Kita, as others have said it is just a comfort thing.....she enjoys kneading into the furry soft belly as it reminds her of her own mamacat and nursing on her as well. Some cats are just inclined to want to do that. It's nothing to be concerned about as long as they're both enjoying the experience, and she's not making Kita's teats sore.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous