When I brought Cleo home, Cinderella was four years old, Cleo was a little over a year old. After the initial separation, they mostly kept to their own sides of the house for the next few weeks. Cinderella wouldn't sleep with me for about 6 months (my bed was on Cleo's side of the house).
Today, they can sleep near each other on my bed, eat next to each other, but they'll never be buddies. But I had the opposite problem you're having. Cleo, the new cat, was very submissive and wouldn't stand up for herself. She's much better now but is still a bit intimidated.
I'm not telling you any of this to discourage you. I'm just saying it takes time. My case is kind of extreme. When I brought the twins home, they were accepted almost immediately, but then again, they were only two pounds. Two tiny wild, monkey-butt pounds of kitty.
Harley's going to smell "strange" after being at the vet, so whatever intro you've done might need to start over. Slang should probably have sole control of the house, while Harley's being kept isolated. It's her world, you're just paying the rent.