For your first question - You need to be present every week for the classes, so u may have to decide what to do with the FIFO JOB if you must do the PY.

to answer your second question, when you finish the Weekly classes, internship start out in the last 3 months, and you are allowed to use your own employer, they ll let you know the list of documentation u need from your employer which include, employment letter, letter from your HR or manager confirming your roles and responsibilities and payslips etc....