My wife and I have a 2 y/o female GSD (spayed) and we found an 8 m/o female GSD (spayed). We introduced them and they seemed to get along in a neutral place (park). I am unsure about how they will get along once I bring the new one home. I have heard a lot of different ideas about female and female, and male and female. I don't want to rush into getting another dog and have it lead to total chaos in our quiet little home.
You are doing the right thing by letting them get use to each other outside of your home. If they get on really well at the park then you could then bring the new dog out in front of your house and see how your other dog does. If there is still good body language going on then you can keep pushing the envelope. However, as in all relationships, a pecking order will need to be established. But you are right in trying to get them use to each other before you bring the new dog back to your place.
Make sure you introduce them on neutral ground at home. For instance, where your girl goes potty, you don't want to take new girl to that place and bring old girl out there to meet her. Does that make sense?
I bet they will get along fine as it is not like 2 males or a male and female. I have 5 females in the house but only 1 male. Everyone gets along fine and they range from 6 months to 5 years. No problems with introductions.
Best of luck!
Friends will step in when the rest of the world steps out.