I think getting a second dog is a great idea. I love having two poodles and love watching the way they interact. See the center picture in my signature below! The relationship that two dogs that live together form is very different from dog park friends or occasional visitors. Some pairs are very close and play together and sleep together. But even those that are not so bonded seem to benefit from learning from each other and having someone else of the same species around. My boy Bob and my girl Sophie (who died in 2011) were more like housemates than lovers. But when Sophie died, Bob really mourned.
Since you like Callie's breeder, I would call and ask her advice. It is hard to know what kind of relationship Callie will form with the new puppy, but I think getting a male is a safer bet. A female would probably be OK, so you could get a girl if you want to. But I have heard that two females or two intact males are more likely to have problems than a boy-girl pair. I know of one case where two female labs became so competitive and nasty towards each other that they had to be separated for life.
When I got Cammie, Bob would have nothing to do with her for about 3 days. But puppies who have just been separated from mother and siblings really crave canine companionship. Cammie absolutely adored Bob and wanted his friendship more than anything else in the world. After 3 days, he permitted her to curl up next to him (the first photo below was literally the first time she was permitted to lie down touching him). After about 5 or 6 days, she was crawling all over him! Photos below show the progression. My advice is to leave it up to the dogs to work out their relationship. You should allow Callie to set whatever limits she wants. It is perfectly normal for an older dog to snap or snarl at a younger dog, so don't stop Callie from doing that. This is how the older dog lets the younger one to stop being so annoying!
Best of luck to you. I think you are really going to have fun with this!