I have a sheltie and she is the absolute love of my life. You talk about happy when mommie gets home. You would think that she was being beaten as much as she "cries" when I get home. I can gaurentee that you will never be met with as much happiness and love from anyone as you will with your dog the min you get home. But I think that you alread know that with 2 big dogs at your home already. I totally agree that I would go with a rescue group first but as you say if you can't find the right dog for you then go with a reputable breeder. The reason I say reputable breeder is that my mom just got a chichihua in July of last year and fell in love with Cassy and got her and the breeder didn't tell her that the little girl had some issues. She was brought back once after the first buyer didn't like her or something, not really sure of the reason. But anyways she is now really coming out of her shell now and is a real looker and a real sweetheart. I would probably really recommend a chi as my moms little dog is only about 3 lbs and will not get any bigger either. I think that most chi's are only about 2-8 lbs at most and are great companions.
As a side note if you are getting an older dog from the breeders please make sure that you ask them how they were potty trained. You don't want to have to try to break bad habbits, it is not easy. Talking from my moms experience.
Hope that you find the right dog and breed for you.