Nina - Wait until you are enrolled with the Uni and have an idea of when your placement dates will be before you contact the hospitals. They won't be able to make any defenitiv decisions until they have the dates. And yes, I wouldn't try to negotiate other placements until you have the dates and know you'll be studying that subject. I.e. I wouldn't try and negotiate 3rd year placements until I had enrolled in 3rd year subjects.
Some hospitals do have arrangements with various Uni's and some have contracts saying they have to take a certain number of students per year. However, most hospitals take more students than that. It is best to just call the hospital you're interested in and say you are a first year mid student with the UniSA (once you have enrolled) and have to do a placement from dates A to B and would really like to do it at their hospital. Be prepared to answer (even vaguely) the kind of objectives the placement has...

To find other Tas Students, there are discussion boards for each subject - we always posted a little intro about ourselves at the beginning for each class at the beginning of each semester (even in 3rd year). This was how I always found people near me. You could also approach the course coordinator to see if they can tell you if any other students in your level are living in Tas. When I started, they actually emailed a list but not for 2nd and 3rd years...
Sorry if I'm a bit vague or don't make sense - I've just come off a double shift...

HTH

MG