I'm pretty late to reply and I see you already had many advises. I am not a specialist anyway, but I just wanted to say that a similar situation occurred with Peppito when I brought Jasmine home. He was hissing at her, hissing at us, and finally never never attacked her. He was even hissing at us when we wanted to trim his nails, something he never did before, as if saying "I need these claws, I am the boss, don't touch them". Now they are buddies. They do not sleep in cat piles, but still they like being together. The only thing Peppito still does after 5 years is hissing at Jasmine and chasing her when she is back from the vet. When they have catnip he also tends to be a little aggressive with her. For the rest, all is fine