Restoring individual database from a universal dump
 

If i backed up all data in mysql using i.e

mysqldump -u sadmin -p pass21 Customers > custback.sql

is it possible to retrieve individual databases or user from this dump?

Yes, but depending on it's size, a painstakingly task. You either need to do some crafty scripting or edit it to strip out the portions you want to restore.

In the future, if you have more than one database, backup each one individually to it's own dump.

Actually, Customers is your DB name (see 1st option here: http://dev.mysql.com/doc/refman/5.0/en/mysqldump.html). Perhaps you meant can you restore individual tables?