I noticed that by default there are a number of MARC fields mandatory. For example, what is the 000 leader field used for and why is it mandatory? Should I be using those fields or is it save to not make them mandatory?
if you want valid MARC, you have to use leader. you can see here what leader is: http://www.loc.gov/marc/bibliographic/bdleader.html most of the fields are filled automaticaly, or can be set as default values. if you build different fremeworks for different material types (books, serials, ...) than you can set appropriate values in the frameworks, so librarians doesn't have to care about it.
BTW, are there any instructions available on using the bulkmarkimport tool?
./bulkmarcimport.pl -h will give you all options listed Marijana