Over the years we've had two dogs at the same time, always one who has been around for a while, and one puppy. Same experiences as the guys mentioned above with very few problems.
Presently, we have a Lab/Australian Shepherd mix and a house cat. We had much more trouble intregrating the kitten into our home, as the dog felt threatened for months by this little ball of fur. Now, he's used to the kitten and they play together (mainly thundering through the house back and forth). They drink from the same water dish and the dog loves the fact that the cat is not a "people food" person, so anything that might drop on the floor is his - the cat sniffs it and walks away.
Personally, I think you will have few problems adding a second younger dog or a pup to your home!
Charlie
