Sicy, you have such a huge heart, and within the capacity to love both cats. I know how you're feeling. For 12 years it was just me and my Snoopy. When I got Shane, I felt a combination of excitement and sadness. I was afraid that Snoopy wouldn't be as close to me, or that he'd feel jealous. I will admit that for a short time, (about a month)sort of gave me the cold shoulder. Snoopy didn't seem to know if he was still #1 in my heart, but that was only temporary. Six years later, Snoopy knows he is my #1 boy, and that he will always be special in my heart, no matter how many other furbabies join us. Snoopy is my "soul-kitty" and he knows it.
Unlike Snoopy, Zoey is still young. Snoopy was a senior when I got Shane. From everything you've said, Zoey will be very happy with her new little brother. He'll give her hours of enjoyment and pleasure. I've heard that cats who are home alone while we go out to work, need another cat around to keep them company, or they get bored. I think you've done the perfect thing for Zoey, by getting this kitten.
I don't know why Zoey is breathing heavily. Maybe she does that when she gets excited. My cat Shane does.
Just remember you aren't loving Zoey any less by getting her a new little brother. You have the capacity to love Zoey as much as you always have, and still love the new baby.
Just remember to kitten proof your house.