Personally, I think its something of an old wive's tale that having two female dogs is more difficult or a bad idea. I've had a multi-dog household twice, the first time my family got an 8 month old rescue mutt (maybe lab/border collie) and had a 6 year old Boston Terrier. That was a rough pairing mostly because the rescue dog had pretty much been crated for the previous 4 months because of ringworm, hadn't been trained, and hadn't been socialized very well, and the Boston was dog aggressive. We initially had gotten a male mutt puppy who died of parvo, and the lab/BC mutt was a dog we took on right afterwards more out of an emotional decision than a logical one. At the time we got her, our Boston was OK with her because she was OK with puppies, but she quickly grew up and there were a few low-key "fight" (which I put in parentheses because no blood was ever drawn and no injuries happened and they broke up quickly, but they were fights and not rough play). I also lived with my parents and the now grown up lab/BC mutt when I got my first dog of my own (who was meant to be my parent's second dog to replace the first Boston). She's also a Boston, and we brought her into the house at 11 weeks, when the older mutt was around 5 and a half. The original Boston and the mutt were both altered the entire time they lived together, and the second Boston (my dog) was altered fairly early, around 6 months. That said, I know of plenty of people with multiple intact females without issues, except maybe sometimes around their heat cycles.
In terms of them getting rough/crazy- puppies play rough, and puppies have a difficult time calming themselves down. At 7 months, the GSD female is probably just coming into the age where she may start displaying true aggressive/self defensive behavior towards other dogs, before that age it would be very unusual if she were showing true aggressive behavior. My guess- and it is a guess, without more information or seeing the interaction in person- is it is probably just normal (though still frustrating) rough puppy play behavior.
Personally, I would not have wanted to bring another puppy into the house for that reason, but there's no reason you can't work on training calm behavior in the house- crating the dogs to give them a chance to calm down works wonders, as well.
I will say- intense food aggression at 7 months isn't a really great sign. If she's been guarding for a long time, I would expect it to worsen, and would definitely not expect it to go away on its own. As someone who loves having multiple dogs at once, I do feel food aggression/resource guarding towards other dogs is something I would not want to live with. I would never feel comfortable leaving dogs out together if I knew one was a bad resource guarder/food aggressive. It is something that can be trained out, if you are interested, and with a dog resource guarding at 7 months I would definitely recommend starting early rather than waiting, if its something you are able to afford. This is personal opinion, though, so take it with a grain of salt.
The only other thing I'd be weary of is making sure that the two dogs don't become too reliant on one another. There is enough of an age difference I wouldn't worry about "littermate syndrome" (which can occur between unrelated but close-in-age dogs, as well as littermates), but I would worry that one or both dogs (more likely the younger puppy) might become unable to function without the other present, which can eventually present a lot of problems. Make sure they are being trained separately, walking them separately would be a good idea as well. I would probably try to shoot for at least 3 outings a week (to a park, someone else's house, training class, etc) alone for each dog, definitely at least 2, to teach them that being alone isn't the worst thing ever and won't hurt them.