Omega-3 Improves Your Pet Health

Okay so I have a few questions, and whoever gives a good answer to all of them gets the best answer! I currently have an 8 month old Golden Retriever male named Sunny, and I love him dearly. He is neutered! For the most part he is very well behaved. He knows obedience, lots of tricks, he has no aggression at all, he doesn’t bark, he’s house-trained, etc. The only behavioral problems that he has are jumping (on people, cats, other dogs), and, we can’t leave him alone in the house because he bugs the cats, gets into stuff he shouldn’t, and jumps on the couch, which he isn’t allowed on at anytime. He is crate trained, and we have to put him in the crate when we leave the house. I think most of this is due to puppy hyper-ness, because he has never been allowed to jump, and has never been positively reinforced to do it.
I’m thinking about rescuing a second dog, but I want to know if it would be beneficial to Sunny, or detrimental. This dog is, by most standards, a “perfect” dog. She is an older dog, about 8 years old, and she is in good health. The lady that owns her has to give her away because she started a daycare in her home, and the licensing won’t let her have pets. The dog is extremely well behaved, she doesn’t bark, doesn’t jump, is extremely affectionate, gets along great with kids, dogs, and cats, and is basically one of those dream “family” dogs that everyone dreams about. And you can leave her out when you leave the house! I don’t know her name or what breed she is, because I just found out about her today. All I know is that she is a larger breed, because she’s about the same size as Sunny, who is nearly full grown.
So I want to know, what’s it like having two dogs? Is it harder, easier, or the same? I mean, I’m sure it depends on the individual dogs, but just answer in general. And I’d also like to know, with this dog being older, calm, and gentle, could she have a positive influence on Sunny? Like, could she sort of help teach him to calm down, and have good manners? I know dogs learn by example, so if Sunny sees her being good and calm, and not jumping, would he figure it out faster? We are due to have a baby in September, so I’d like Sunny to stop jumping really soon!
Just to clarify, I wouldn’t be adopting this dog solely to help Sunny, that would be ridiculous! I love animals, and this dog sounds really sweet and affectionate, which is an added bonus. Let me know what you honestly think please! Thanks!