I've owned a female Jack Russell for several years and now I'd like to get another dog. What should I be looking for?
Same-sex aggression and aggression towards other breeds of dogs is well documented with this terrier. It is strongly recommended that no more than two Jack Russells (of opposite sex only) ever be permitted to stay together unattended.
If you do get a second dog, it would be a good thing to introduce them on neutral grounds (if the 2nd dog is just a puppy, avoid the parks and the germs, try a neighbors backyard, where you know the dogs are well vaccinated!). It's important to discourage any show of dominance or aggression from the start. Give positive reinforcement for nice behavior, on both sides (praise or a treat).
If you are willing to manage two females, separately, if need be, then you can consider getting a 2nd female. However, if you are not willing to take the steps to keep both of them safe, (should you find out when your little girl gets older that there are problems), then I would pass. Many owners have multiple females, but we keep those that don't get along (yes, some are spayed and still squabble!) separated when unsupervised.