I have an established pack of two female Lab mixes and one male. The male is the alpha, though he only exerts it when absolutely pushed, and necessary! The second is a Shepherd, Pit, Lab mix and is very docile. Lastly, #3, is a terrier, Beagle, Lab mix. I introduced #4, a Lab, Shepherd mix and who knows what else. It is a female pup 8 months ago.
I kept her separate due to her hyperactiveness. I have been successful with the alpha male accepting and being protective of her. The #3 position pack dog and she have bonded closely becoming daily playmates! The middle pack dog, #2, just won't stop being aggressive towards the puppy, who is actually larger than my other two dogs!
She is a sweet, very submissive, playful 8 month old so I don't get it! Can anyone help me? How can I get my middle dog #2, to stop being aggressive towards my sweet puppy? I try putting them together and the puppy continues to try to play even though the large female is showing teeth, growling, and biting aggressively. She doesn't break skin, but I'm afraid she might! I've never seen her act aggressive in the 5 years I've had her. She is a little territorial with my #3 dog about being nearest to me, but never vicious or biting. What's up? All but the puppy are therapy dogs. The puppy is in training.
You already had some problems.
You say your #2 is "a little territorial" with your #3 about being near you. This is jealousy, plain and simple.
Then you say your alpha can be aggressive, but you make excuses- only when he's pushed. Well, maybe, but this is reactive, not proactive. He isn't showing great leadership skills. Sounds like he is not a "true" alpha or the firstborn in his pack.
Your dogs had a pack going that they had worked out through a rather careful balancing act. Then you added a new female. The female will grow up to challenge the second as top female, and she knows it. The male doesn't care who wins, because he's the winner with either of them. And the third doesn't challenge the young one because she's not the top female anyway.
The problem with dogs in your home is you can't let them act like dogs in the wild. You must be alpha. Any dog must be beta. You have to be the leader. If you are truly the leader the dogs will respect that and listen to you, not jockey for position.
Your dogs and you need pack leader training and it should have been done before the fourth dog entered your pack. It would have been easier to establish your position then. But pack leader training is what you need.