Mine's a shelter dog too. We got her at two and a half. She was in there for the second time because she attacked a smaller dog (both times), though we think the smaller dogs provoked her because she's got scars all over her legs. Since we got her, we've noticed that she's fine with any dogs her size or larger, but anything smaller is automatically a target to her.

It makes having that Siamese I've always wanted a little difficult....