There is a good method to taking it slow & careful. A gentle dog should fit in well-eventually.
Five days ago I got a young dog. I already had three cats . I’ve let the cats run loose at night and kept the doggie in the bathroom. I let them all sniff under the door. I’ve taken the dog with me in the daytime and let the cats be able to go into her bathroom to check out her scent. I’m not comfortable letting her roam the house yet with the kitties. The cats are not hiding and are circling around staring at her when I bring her out on her leash in the house. She appears to want to play; they, of course, are disgusted…but also curious. I need to let the cats get nose-to-nose soon with her on the leash in case any fights break out. Do I interfere if they growl/spit? Just hold the leash so they don’t chase and fight and talk soothingly? I talk to them all when they’re in the same room. I also shut the cats away in the morning and let her roam the house where cat scent is. None are ever loose in the same room. The dog doesn’t bark or growl other than a small woof each time. She seems to want to play and makes a “lunging” movement that appears playful. I expect this routine to last a few weeks at least. Then I’ll let them all together only when I’m supervising with the dog off the leash. Am I doing this right?
I’m impressed. You have a fundamentally sound approach. You’re basic understanding of the fear felt by your cats and the curiosity and playfulness of your young dog are on the mark. I’ll make a few recommendations that will help the process along.
Number one: To your group of cats you have added two stresses. One is another creature in their already crowded household. The other is fear. The results of these new stresses may be physical disease. Or they may act it out with behavioral problems like urinating on your walls and furniture. They may also fail to accept this new family member-permanently. But let’s work on it anyway; we still have a shot at a negotiated peace settlement.
First eliminate fear. Sit with your dog on a leash at one end of your house. Give her a rawhide chewy so she is not focused on the cats. Then let the cats out to explore the house with your dog visible. Repeat these sessions until everybody appears completely bored. Then: The final frontier. Get a twenty foot light weight nylon cord at the hardware store. Sit with your dog with no toy and have the cord tied to her collar. Allow the cats to roam the house. Any aggression by your dog is corrected with a sharp jerk on the cord. As her behavior improves allow her to leave your side but always with the nylon cord attached. Thus the cats will learn not to run from her and she will not feel motivated to give chase. Lose the cord when you have confidence that this youngster fails to notice the existence of the cats.
Will it work? Only time will tell. Most dogs do well this way. Some are aggressive by nature and can’t resist the hunt. And that can be deadly. Use your head. If your dog keeps failing the test you better leave her outside.