Adult eating kitten food

Hi all, so i recently rescued a kitten who is eating solid kitten food. My other kitty stella was very underweight when i got her and iv been trying to get her to gain some weight with no success until i starting putting kitten food out, i noticed she was actually interested in what the kitten was eating. She has gained alot of much needed weight. I have tried so many different flavours and brands of adult cat food and shes never been to bothered about any of them. So now im wondering is it ok for her to keep eating the kitten food? Or will she keep gaining weight, i dont want her to turn tubby but i am very happy that she has finally gained some fat. I have been trying to fatten her up since april and didn't intend on her eating the kitten food. Now that shes reached a nice weight should i stop her eating it? Even though she hates everything else? Thanks

Check the AFFCO seal many kitten foods say for all life stage or for growth and maintenance if it very much okay to feed for a lifetime ... if it says for growth and lactation discuss it with your vet