Background, we moved into our house in December and became friends with a stray in the same month! Previous owners of the house have never seen the cat/were dog owners, so cats coming into the yard was rare so we assume there's little to no scent on the yard. In January, we learned that he was originally an inside cat for our neighbors grandparents. Later the grandparents couldn't care for the cat, so they gave it to their daughter, the mom, who doesn't like cats, so she just let the cat do as it pleases and doesn't really care for the cat or let him inside anymore.
Present, since then we've slowly been trying to integrate this stray, Socks, into our pack. He's such a sweet boy and we feed him about every other day in our backyard. Eventually, he started self-inviting himself into our house for feeding time. Which is fine, he has manners for a cat. He has no fleas or ticks or any open wounds since the time we've known him. His only problem is that he's dusty.
Our cats, our oldest cat, about 13, Estella, really has no ill will against Socks besides mean glares. She eats, she watches, she leaves to her room. Our youngest cat, about 5, Ezekiel, for some reason has a vendetta against him. It was a shocker to seem him chase and swat Socks around. Ezekiel is a really friendly and sociable cat to people and other cats. He travels with us and is fine with my families' other cats. When he's visiting, he never starts fights/negative chases, he's very humble and submissive to the cats who do. Same applies for our pervious neighbor's cats when Ezekiel was allowed outside more, no fights. Both are fixed and neutered and up to date with shots.
What we've tried, swapping scents usually done by petting or blankets. Eating together at a distance, 9-10 feet away. Playing simultaneously, although Ezekiel LOVES his toy, he can never keep his interest when Socks is inside. Socks also shows no interest in playing. Playing beforehand, Ezekiel can play forever but still never tired enough afterwards to not focus on Socks. Affection and treats simultaneously, maybe 1-2 feet apart. Smelling scents through door/mesh.
Other, we do PLAN on asking Socks's owners for permissions to house the cat if the cats can become compatible. Socks does start swats occasionally, consider it 20% him and 80% Ezekiel. We do let Socks into the house when it's hot to hang out in AC under our supervision. (100 °F+ / 40 °C+ is a nightmare.) Socks also occasionally wanders and roams the house when the cats are in their rooms asleep.
What else can we do? Should we just give up? It's been months since we've started trying to get them to be in harmony. Although they rarely have scuffs, there are lots of growls and stalking. Thank you for reading! ^^"