steps to ensure referential integrity

I have inherited a database that was originally created in postgres version 6.? There were no steps taken at that time to implement constraints to ensure referential integrity. There are no FK constraints, PK constraints, etc..
Most of what currently exists is done in the existing code. I would like to change this and am interested in getting suggestions. I expect that PK constraints on each table as well as all FK constraints should be in place. Anything else that I should consider?
I have indexes currently set up on a variety of fields, including PK fields. Do the constraints automatically generate indexes on these fields such that I should remove the ones created at the end of my script?
Thanks in advance for the suggestions!
Jodi