I think the woman at Petsmart has a point about the male/female combination. I hate to disagree with Jen, and obviously KAK and her both have happy large households of males, and I had two males before I got my girl, but statistically, you will increase your chances that they will get along if you get a boy and a girl. Or two of the same gender, separated by years of age. (5 years)
These are the guidelines of Siameserescue.org whom I foster for. Check the site out for info on cats.
And if you can be patient, and isolate the new cat for 10 days you will be rewarded. It's for the protection of your own cat, so he doesn't get sick. Many/most cats from shelters will come with a illness, both mine did, it's a part of rescuing a cat. Even Petsmart can't guarantee the total health of their pets. So don't worry if she catches cold, just get her to the vet for care.

Have a happy day picking out your kitty.