Ahhh, one of my favorite discussions. I made a leap of faith bringing home a GSD puppy when my golden retriever was almost 12 years old -- she had always been an only dog, just her and me. I couldn't have imagined it would turn out better than it has. My golden has been an amazing role model for Spirit, she makes it so much easier to raise him. He adores her and she's much more active now.
There is some jealousy but oddly enough it's with Spirit, not so much Daisy. Spirit would take all the attention, and the food LOL ... I have to be careful to not let Daisy feel sidelined. It is a fact though that Spirit, being a puppy, requires more time and attention than Daisy does. I struggle with this a lot.
Daisy's top dog though, even now that Spirit is 8 months. He hasn't seriously challenged Daisy yet, though Daisy has corrected him more than once.
You ask how getting a new puppy affects my relationship with Daisy -- it has made it better. I'm grateful for Spirit -- without him, I might never have seen this side of Daisy. I know her better now, and I love her even more.
But I want to add, I was very careful in my selection of a new puppy to bring home. I didn't go about it willy-nilly. I brought the pup over for play dates several times before I made the decision that this would be Spirit's home. I saw qualities in him that I liked, I see them still today. Spirit and Daisy are a good match, I do take quite a bit of credit for how well this has worked for all of us