Because my food is not grain free and they refused to give me the cat unless I agreed to not change over his food and switch hers.
Our vet said not to feed our cats grain free food. She said new studies are showing that more cats are getting sick from it. I don't know if that is true I haven't read the studies though.
I introduced 2 new cats in to my home back in October. My other cat did not take well to them. She doesn't like them at all, but she does tolerate them and has learned to co-exist with them.
I tried not to disrupt her as much as possible when bringing them in. We kept them in an area where she didn't use, and kept them out of all bedrooms since the other cat spent most of her time in either my dd's or my ds's rooms.
I would have never agreed to changing her food (not saying you were wrong too) because that would have just meant more stress.
I had a vision that she would become this loving and nurturing cat to the new kittens but it never happened. While I wish they were one big happy family I'm just happy there aren't cat fights constantly. I don't see her ever getting along with them, only tolerating them.
I don't think 2 weeks is enough, it took my cat up until around Feb to be able to walk by the other cats without hissing and going in for a swipe. Now she will stay in a room with them, but doesn't like if they come near her, she will go near them but not the other way around.
Give it time, but be prepared for the reality that they may never get along.
I also wouldn't risk my cat's health though, I would let her eat her own food, just keep it separate from the other one. If she continues to be too stressed with the other one there, I would find a new home for that rescue. Good luck.