When my Yorkie, Gracie, came into the family there were already two Chihuahuas, and four cats (3 females & 1 male). From day one Gracie has adored the cats. All of the cats are double or triple her size. Its adorable to watch, whenever she sees any of the cats she wags her tail so much and so fast the entire back half of her body wiggles. At first the cats were a little apprehensive, but within a week they were OK with her. Some are move lovable - but none have ever attempted to hurt her. If they get their fill of Gracie, they will just jump to higher ground to "take a break." I don't know if I would trust other cats around her though.
P.S. The Chihuahuas love her too !!!
