Hi, I'm Ben and I'm studying languages as part of an Arts/Laws degree.
Changing subjects goes from dead-easy in the first three weeks of term (minus the catch-up work) to progressively more difficult. I'm pretty sure you have three weeks to change before there are any consequences to changing a subject, e.g. you might have to pay for it.
Don't do Russian at ANU - they only offer it 'for academic purposes', i.e. so you could read historical documents on the Soviet Union or something. Google 'Russian ANU' for more information. You won't be learning anything like conversational Russian. You might be able to study it properly at another institution in Canberra alongside your ANU course, but you'll need to speak to ANU Admin about that...
Also, please note Chinese makes you take two courses for the first year (Written/Spoken.) This would constitute half of your timetable, and possibly slightly more of your overall workload as they are amongst the more demanding Arts/AP courses.
I don't think there are any 'catch-up' courses offered either during term or in the holidays. You'd just drop the language and start a new one the next year. Don't worry if this means extending your degree - a lot of people do it. Your uni years are your best years anyway, and no one cares about your age when looking for grad jobs unless you're like 30+ or something.
For IR, I'd advise taking Mandarin. While French is more prevalent in IR-related institutions around the world, Australian institutions (i.e. DFAT) are desperate for Asian language majors. Australia has plenty of francophiles like myself without anywhere near the need to compensate. French would only trump Mandarin in utility if you were to work in Europe or the States where many of the aforementioned international institutes are situated.
That said, you might find Mandarin difficult. Most agree that it's one of the hardest languages for Westerners.
Let me know if I can be of further assistance!