I have two VERY large cats (19lbs & 23lbs). I was so worried about how I was going to introduce my Silvia to them, especially since she was so tiny (only 8oz) when she came home. I remember laying Silvia on the floor in front of me while she was asleep, and letting the cats smell her. Well, she moved ever so slightly and within a millisecond, those cats were on their hinds popping her repeatedly with their front paws like they intended to kill an intruder!
They're also declawed, so it didn't hurt Silvia, but it scared the crap out of me. I remember just crying and crying because I figured they'd never get along. Well, it took some time, but they are inseperable now and it's like they've been together forever.
Oh, and another thing you can look forward to in raising a yorkie with cats is that it will introduce you to the possibility of raising a corkie! (part cat part yorkie)
My two cats essentially adopted Silvia as their kitten. Silvia lays on the back of the couch with them, sleeps with them when I'm not home, and even cleans herself like them. The first time I caught her with her back leg in the air licking it clean, I thought I was going to die of laughter! She also NEVER barks, but instead kind of moans, almost like she's purring when you pet her! Poor confused little thing!