It all sounds like normal cat behaviour and hissing is just a cat expressing itself, afterall cats who are best mates hiss at each other from time to time.

And time is all it will take, you've dome it all right so they should be fine. I was just saying to my husband that it's taken 6 months for one of our cats to fully accept the puppy and they were cuddled up today - we took some photos. So give it time I'm sure they will sort it out

Feeding is great to do together get the treats out and feed them together if you can. They really bond that way. Tuna or something when you think they are ready.

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Mummy to 1 puppy, 3 cats, 3 chickens, 2 guinea pigs and 2 small children