Its VERY cool that you rescued her ! Its completely normal for dogs to growl at eachother, it would actually be somewhat wierd if they didn't growl at all. Its just a vocalization, a lot of dogs even growl when they play, and its usually a warning - it is MUCH better to have a dog that will growl, as if to say "leave me alone. .", than a dog that will snap and snarl immediately when angered. You should try crate training her, and your male Beagle. That way, they'll each have their own crates, like safe havens to sleep and hang out in away from each other. I would not totally worry about the growling. . its like a middle-aged person scolds a teenager, she's probably just telling him to chill out. You SHOULD start worrying if she injures him, but they might go after each other a few times while they're testing one another - as long as there's little or no bloodshed, scuffles are pretty much normal especially with an older dog and a younger dog.

It actually sounds like the female is more dominant. . . which is the usual. Personally, I would allow the female to be the "alpha" of the two, since she IS older and she IS a woman . . going through Obedience classes with her and training her on a daily basis will also establish that the people are the "alpha's" of the house, and they both need to follow your rules.