I had the opposite problem when we got Zak. I had an 11yr old mini-wire (Muffin) who didn't want to play with a 6 month old big GSD puppy! I used to put Muffin in a cage when I left them alone. I just wanted to make sure that Muffin could get away from Zak when he needed to. After a few weeks they soon settled down. They will learn to get along don't worry. Tilly (terrierX) really aggravates Zak to death, but he is brilliant with her. He only ever shows his dominance when we is eating his chicken wings in the morning. She can bully him to leave his dried food, she barks at him & he walks off & leaves it. He only gives a low growl to her when he is eating the chicken & she stands near him, but she won't look at him or challenge him. She tries to throttle him while they are playing, she very often bites his lips & jowls. They very rarely get too rough though, it's all play.