Advice needs: kitten adoption and resident cat not happy

My foster dog's owner had given his two dogs into foster care by friends of his for at least half a year each time he planned to travel somewhere for longer. Once it was for a whole year and while the one dog first barked at his actual owner before getting to sniff his scent and remember him, the two dogs had had quite some issues with getting along with each other again after a whole year of separation.

Their owner decided to try and fixate them each to one side of him sitting on a chair. They would both look at each other and of course initially bark at each other a lot. But step by step the owner conditioned both of them to stop barking at each other by giving treats, which led them to growing together again.

I don't know whether cats might be more complicated here nor if the two dogs remembered what they got along well at. For example your resident cat might accept eating together with the kitten, but not sleeping in the same room. Or sharing toys or whatever. But if you try and maybe succeed in conditioning to at least tolerate each other doing one of these many things, you could try and repeat this for whatever doesn't work by then.

Buy a Feliway plug in. The plug in floats calming soothing cat hormones into the air.

Resident cat has been neutered. Kitten also female and waiting neutering. I hope with time.
Resident cat was rescued so not sure of her history but she did spend a lot of time at the vets and In a cattery. I feel so guilty introducing kitten but she also needed a home.
It’s just taking such a long time and I don’t feel we are making any progress
Thanks for your comments.

Introduced a younger spayed female cat to an older very mellow neutered male cat. Female was a pill around that male until he died. Not big fights, but hissy fits.

She dislikes other cats. Period. She had had several homes after shelter, so think it was some trauma.

Has the resident cat been neutered/castrated? Other than that try rubbing each others scent over everything in your home and keep the mesh gate there to ensure both the kitten and resident cat can observe each other at all times. They will eventually tolerate each other unless the resident cat has been through some severe trauma, in which case it maybe practicable to consult with a cat behavorist.