My only experience was probably 30 years ago ... I had a small Peke-a-poo and Brandy was the bossiest little thing!! I bought my son an AKC Collie puppy for Christmas and she was adorable and smaller, at that time, than Brandy. Brandy would smack Duchess [Collie] around and just acted like a "bully"!! In case you all don't know, Collie's grow pretty fast and it wasn't any time at all before Duchess turned the tables on little Brandy and showed her who was alpha!!
They got along really well but those first few months were very interesting!!
