If you plan on having both sexes, make sure to spay/neuter everyone. Mixed-sex groups are awesome to have and a lot of fun, so it's worth the extra expense to get them all sterilized. Plus, girls are extremely prone to mammary tumors and infections of their reproductive organs (such as pyometra), which spaying either greatly reduces/gets rid of.
Also - remember that getting a female from pet store is a huge gamble, since you might end up with a pregnant mom. Some rats don't show they're pregnant until literally a day or two before they give birth, so you can never tell just by looking at them!