Well, I have a BC mix named Jewel that I got from the shelter and a GSD. It was really rough at first as Jewel tried her best to be Alpha dog when she first got here. I've done a lot of work with them both to get to the point we're at now.
I would suggest you get a puppy if you already have a doggy that's been with you awhile.
Yes, I agree that two dogs are better than one for you and for them. Titus is so much happier now that Jewel and he have gotten used to each other. Before I got her, I was his only playmate and he has a ton of energy. I was exhausted trying to exercise him.
I now take them for walks separately and train them separately and together. There have been no problems with them forming their own pack. I'm definitely part of their pack but I'm also hanging out with and interacting with them all the time when I'm home.
I will never have just one dog again. I think they really appreciate the company when you're not home and it makes them feel more secure too compared to dogs that are by them selves while their people are away.