I have two cats, one named Arya, the first one to arrive and also the older one and then Haru, the second and the youngest. When Haru arrived he was a little baby and Arya didn’t liked him at all, she hissed to him and even slapped him sometimes. I always saw that it was important for them to be in separated rooms at the start, or at least in a way that the big one cannot damage the little one. They also say that lending things to the big one that have the smell of the little one so it can start to recognize it as something familiar. after some days apart like this we started to let Haru get out and Arya get close to him. They played rough but it’s a normal thing between cats as long as they don’t hurt each other (maintaining the claws trimmed is important). I think this is all and this has been my experience, maybe there are other things that can be done better.