Its odd. Getting a second dog was a huge adjustment. Getting the third and the occasional fourth(rescue foster) is not as bad. Your original dog is older, better set ways and where your want her. So that makes it easier. I think its a great time to add a new dog. Right now all my dogs are about 3-4 years apart. I think thats perfect.
But remember to not forget your older dog. The puppy will need lots and lots of attention. Make sure you dont forget your adult girl. She is going to need one on one time, training and love as well.