Hi,
I stayed with my husband's grandmother for 6 weeks with our 9 year old Yorkie. My g-in-law had a psychotic black cat that had a real personality disorder (1 min. loving and friendly -- next very seriously biting your leg :( ) Pepper, my Yorkie, hated and had fought barn cats where we had been living. She even tangled once with a Bobcat that reached out of a cage and grabbed her. (My arm suffered from that encounter :( ) Cats were the enemy to her, but I didn't want to upset my "new" in-laws :)
Within a day staying there, Pepper learned that Ebony and the other one (very shy) were completely off limits. I put her on a leash and every time the cat walked by, I told her to "leave it" and "sit". A couple of days later, Pepper stayed off of the leash permanently :) It worked for her :)
Now I did allow her to be proactive when Ebony walked up and got within about a 16 inch space from her, just for safety's sake. Ebony was about 18 pounds to Pepper's 7.5. And very unpredictable. I would "ssst" the cat to keep her away from Pepper. Ebony would taunt her deliberately, but Pepper ignored her antics.
No fights happened during the entire time we were there, so something must have worked :) There were little chases from Ebony getting too close, but it was controlled bursts :)
toodles