I have had success with two females together. It really just depends on the dogs. In my opinion, gender is less important than overall compatibility and that can be really hard to predict.
Our oldest dog is a female Doberman pinscher named Gracie. We got a female labrador retriever named Luna when Gracie was three years old. They are an odd couple, because Gracie is very high strung and more than a little neurotic. She is sensitive about touch and confined spaces. She is also an intense and focused dog that tends to take everything very seriously. Luna is not. She is a laidback goofball and quite relaxed. Somehow they work well together and play well together. I think it helps that Luna is very quick to apologize if she upsets Gracie with her silly antics, but equally quick at going back to being a shameless clown.
More recently, we have added a third dog, this time a boy. I don't think the gender would have mattered either way with these two dogs. Luna loves everyone equally, no matter age, size, gender, or species. Gracie is a nervous wreck when meeting new dogs, regardless of gender, but she adjusts relatively fast once she gets to know them. Neither dog has a commanding or assertive personality and neither one has shown any female-female aggression.
The new addition is a very big, very mellow Anatolian Shepherd with a stoic personality and independent nature. The three dogs couldn't be more different, but they all get along and enjoy each other's company, so I'm happy.
You can see their personalities quite clearly in the way each one "relaxes" on the couch:
And yes, Luna actually sleeps in that position frequently. She opened her eyes while I was taking the picture.
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One thing to keep in mind if you do get a puppy is that young dogs have a lot more energy than older dogs. This can be quite exhausting for the older dog and you will want to be sure to monitor interactions and separate the puppy if he/she is bothering your older dog too much. If you don't do this, your older dog will need to correct the puppy for you, and that can lead to problems. Adopting an older dog might help reduce the age difference.
However, Aussies tend to have a lot of energy in general. And getting a puppy can provide a smoother transition from a one dog houshold to a two dog household, because a young puppy is a less threatening intrusion compared with the sudden arrival of an unfamiliar adult dog. Raising a new puppy is a lot of work, but also quite rewarding and it allows you the opportunity to work with the dog from a young age so you understand their unique characteristics in depth as they mature and can shape their progress. In contrast, adopting an adult dog allows you to see the finished product while skipping over the intermediate steps. It will be harder to change anything you don't like, but if you find the right dog, it could be a perfect match from the start. There are pros and cons to either path.
