So you think it'd be safe to get a girl for our dominant male? Salty has a Napoleon complex, and likes to bully smaller dogs, but he respects big dogs.
Yes, Salty will probably rule the roost a bit while she is a puppy, but as she matures, she will gradually become more confident, and challenge his bullying tactics.
Males usually give way to females in the dog world. Letting her have her first heat might be a good idea, even though many of the vets advise against it. It goes without saying that you never leave her alone, or off leash at this point in time, and spay her soon afterword.
You need to establish yourself as the alpha figure in your pack, of course. You are the one who rules the pile. Then, they decide who comes next in the "pecking" order. You respect that, making sure that you feed them separately, and preferably in their crates. Expecting them to sit before you hand out any treats is also a good idea, and giving the dominant dog the first treat establishes and reinforces the "rules".
It is not a good idea to intervene in dog squabbles, usually. I would be watchful when you bring the new puppy home, and might ask the breeder if you can wait until the puppy is about 8 weeks old, at least. The adult dog will have had a chance to teach the pup some basic "dog" manners by that age. Some breeders will send them off a bit too young for those important lessons to have been taught.
When you bring the new puppy home, do not favor her, and spend a bit more "quality" time with Salty alone than you might tend to do. Crate train the puppy immediately, and when she is driving Salty nuts, put her in the crate for a rest. If Salty gets too agressive with the puppy, send him for a time-out in the crate immediately, with no comment. He may need some peace and quiet, too.
There are many advantages to a two dog household, and I always advise friends to have two. It is best to get one first, establish a good relationship with that dog, then get the second dog. They can play together, excercise each otehr, and do doggy things together that we two-leggeds just do not quite do as well.