When Sophie Kat came into our lives and household in June, we worried that our two Border Collies, Grace and Sam, might not accept her.  We gave Sam (who is more aggressive) a job: "Guard Sophie!."  That worked like a charm, he was gentle with her and loving. 
 
HOWEVER, Border Collies tend to take things one step forward.  When Gracie showed any interest in Sophie, Sam would bark at Gracie and get angry.  We had to change the command to:  "Love Sophie."   Border Collies are so darn smart that they can distinguish between the nuances of words.  "Guard" means keep safe and "love" means to cherish.  Sam figured the differences out in a day or two!
 
Sam and Sophie are famous friends, but also let Gracie in on some of the love!