CSV Import Manipulations

I am preparing to import a large CSV file and have some preliminary issue to plan for which I need some experienced advice for:

Can I test import a small part of the csv file
How does the import process know which fields go where? Ie How do I map fields?
Can I delete the entries only appertaining to the import if there are errors?[thereby leaving any existing listings intact]

This will be most helpful since I cannot afford to corrupt the existing content.

Is there a simple way I can let my client know how to structure the CSV file that he has to produce to make it easier for me to import into my database taking account of all my Custom Fields?