My fiance and I have had a girl cat for 5 years now, living in his parents house for a couple years where there were cat and dogs. She was fine with them. Then we moved out to an apartment w her alone for the past 6 months. We decided we could get another. We got him yesterday and kept them separated the first night. There was hissing and growling. All night and this morning he wanted to go out and explore so we are letting them out in the living room together. They aren’t fighting but hissing and growling. But not swatting at each other or literal fighting. Am I doing wrong by my established resident cat? Right now she’s loaded on the counter and he’s sleeping on the chair next to her. She’s just staring at him.