Most people incorrectly use the terms dominant and dominance with regard to dogs. Here is a good blog post by Dr. Sophia Yin on the topic.
http://drsophiayin.com/blog/entry/dominance_in_dogs_is_not_a_personality_trait
I got a 15 week old puppy when my female was 8 and my male was 5. Both were happy to have a puppy around. They all spent hours chasing each other and the squirrels in the back yard around.
Fortunately my puppy didn't pick up my female's habit of digging up the back yard.
