Megan may be different, but I've noticed that Buffy tends to gravitate toward dogs that are her size or smaller. Her best friends are beagles, cockapoos, other Shelties, and various types of mutts. If a dog is too high energy or huge she is more cautious (although she got along famously with Kirby the Lab-poodle mix when they stayed together for a few days).
The thing is, it depends on the type of dog. I think you'll probably want a Lab or Golden, right? Then get that breed if that's the personality that fits you. If Megan's anything like Buffy she'll let the other dog know when it's getting out of hand. I think ideally you will want Megan to meet the new dog when you do, or perhaps do a trial to begin with to make sure the personalities will work out. Just like some Shelties are barky, some are skittish, some are lazy, and some are easygoing, other breeds are across the spectrum too. I think focus on a breed that works for your family and then look for an individual dog that works for Megan.