I don't know if this really means anything. But, I had two Maine coons and I know they would never have let any other cat in the house, but perhaps a young kitten or 6 month old. They always seemed to treat a young kitten or 6 month old different than an adult cat. Maybe it is a behavior that helps keep babies alive among other cats.

Now, my two maine coons are gone. So, I adopted a 5-6 year old black cat that hates other cats, fights with every cat in the neighborhood! However, I brought a found stray about 6 month tabby into the house and he accepted him, even though reluctantly. I do not think he would ever have accepted a full grown cat.

So, maybe try a young 6 month cat, slowly get them used to each other and maybe it will work