Process for populating tables in new database

I have a new database with five tables, and text files with data to populate
the tables using the insert command.

1. Each table has a sequential primary key. Should I manually add keys to
each insert row or will postgres generate it automatically? Example, for the
companies table:

org_id    | integer               |           | not null | nextval('companies_org_id_seq'::regclass)
org_name  | character varying(64) |           | not null |

2. Should I use sepatate files as input to each table or can I combine them
in a single file? For example, adding a company name and contact name for
someone in that company using input commands for both tables in a single
file? I will need to provide the primary key to the company table as the
foreign key in contact table and have not before pupulated multiple related
tables in a new database from .sql files.

TIA,

Rich