Yes, your student ID is your login as well. You need it to login into your WebMail, My Student Admin, UTSOnline and some other university things. Your e-mail address is either your
studentID@student.uts.edu.au (though @uts.edu.au will work until 2006) or your
FirstName.MiddleInitial.LastName@student.uts.edu.au (or no middle initial if you don't have one).
Remember this is your UTS login. Some faculties (such as the IT faculty) will also give you a faculty login account (and e-mail as well) for you on their computers, so this will be seperate to your UTS one.
As for late enrolling, if your late enrolment session is after the 21st of February, then yes you might miss out on some preferences, otherwise no. I advise you try and make it you first enrolment session.