I became involved with GSDs in the mid-90s and my last GSD passed away about a year ago at 14 years old. I had 3 GSDs, one German showline girl, an American line girl and an American line boy. I had and still have many friends who are involved in GSDs of various lines. Out of the GSDs I had, two were same sex aggressive (SSA), the GSL girl and the Amline boy. The Amline girl was not really SSA but was very willing to seriously fight if things went that way. Most, if not all of the GSD people I have known have had experience with SSA regardless of lines. So IMO and IME same sex aggression is something to keep in mind with the breed for sure if you are someone who enjoys having multiple dogs. SSA is the reason I don't have GSDs any more. Since you have two females already, I would strongly suggest that you do not get a female GSD. A male would be a much better/easier fit for your current dogs
Another thing to keep in mind is that they can be very serious guard dogs. By that I mean, many owners have issues with their GSDs being too protective of their house, yard and/or car. The breed needs a lot of proper early socialization, training and on going proper management so that their protective instincts do not get out of control or cause them (and you) problems.
GSD can be great dogs though, they are a breed that you have to really know to fully appreciate because most are so owner-focused that they don't always seem that interesting to people just seeing them out and about. They are intensely devoted to their owners and very "task oriented" in that they really want to do stuff for and with you. IME they tend to be very intuitive in training once they are bonded and understand the training system, they sometimes seem like they are thinking in the same direction you are when teaching them new tasks. They can do well at tasks which require them to be intensely driven and powerful then turn around and be just as good at tasks which require they problem solve and work carefully. Just really smart and willing dogs. At least, that's how a good GSD is. You will see a lot of examples of not so good GSDs too though and even among good GSDs, there can be a lot of variation. Research, research and research lines/breeders/work/sports/etc and so you can find the right breeder for you and your needs.
ETA: I just saw you were interested in adopting. I'd suggest going with a GSD knowledgeable source and getting a dog who's been in a foster home so you have the best idea of the dog's temperament and if they will be a good fit for you.
As for health, I don't think a random GSD mix is going to be any more or less healthy than the average pet-bred GSD that ends up in rescue.