If you have one very submissive female and one very dominant one - and they're both already adults - then you've got a reasonably good chance of them getting along. Your trial weekend should give you some clues.
On the Australia thing though - there is no need for her to enter a 5 month quarantine. The minimum quarantine period for a dog imported into Australia from the US is 30 days, not five months
Of course, that does rely on a fairly standard process being followed before
the dog leaves the US. She must first be microchipped, then vaccinated against rabies, then a blood sample taken/tested, and then after 6 months (180 days) may travel. That is pretty standard for travel to rabies-free countries.
Obviously, it still means there is a six month wait before the dog could join her owner in Australia. But there is no need for her to spend that time in quarantine - she could just as easily be dog-sat if there's anyone in the US willing to do it. More information on Australia's quarantine and importation requirements can be found here: http://www.affa.gov.au/content/outpu...ntType=outputs