You can get some form of FAA PPL, either stand alone or dependent on your Oz PPL. You must meet the minimum experience requirements for the FAA PPL and jump through the TSA insecurity bull****.
You can train for a CPL too, if you wish. Still have to meet the FAA minimum experience requirements and satisfy the TSA's crap.
The FAA PPL, CPL & ATPL are segregated by Category & Class. Each segregation in a licence stands alone in terms of the privileges and doesn't embrace privileges of another category/class combination. Each category/class combination requires a flight test. If you already hold a licence then the test will be an 'add on' checkride so common items need not be tested again although an examiner can require *anything* from the Practical Test Standard for the licence.
'Category' includes rotorwing, airplane (sic), lighter than air and the like.
'Class' distinctions for the airplane category are Multi Engine Land, Multi Engine Sea, Single Engine Land, Single Engine Sea (MEL, MES, SEL & SES, respectively).
You could, for example have a PPL valid for SEL, a CPL valid for SES, and an ATP valid for MEL. You would get PPL privileges in the SEL, CPL privileges (which includes PPL privileges) in the SES and ATP privileges (which includes CPL & PPL privileges) in the MEL.
As for single vs multi instrument ratings, every country of which I'm aware segregates multi IRs from single IRs. They all allow a multi IR to be used in a single and disallow a single IR to be used in a multi**.
**Some countries have an exception to the rule that allows a single IR to be used in centreline thrust multis eg Cessna's C336/337 if the pilot is already qualified to fly the a/c VFR.
Last edited by Tinstaafl; 2nd Oct 2009 at 04:21.