If your 13 year old female dog is well socialized and seems to get along well with other dogs, I don't think it should be a problem. Make sure that when you do bring a new dog into your family that you take your time to introduce the two dogs to each other very slowly and methodically. Make sure each has seperate eating and sleeping areas. You can start by getting the new dog a crate and letting your other dog sniff her out. Do this for a little bit and build her up to more depending on her toleration level.
We brought a young Jack Russell into our family that just adores our older dog (a mutt) and even sleeps on him. Our first concern was our older dog and like I said in the above we built up their time slowly - especially because the Jack Russell has so much more energy then the older mutt. Now they are best of friends.