great that you want to use eZ publish as a forum. The forums here on ez.no is of course an example of a forum using only eZ publish.
To get the permissions correct every user should be able to read a forum topic and reply. They should be able to create both topic and reply but only edit topic and reply where created by themselves (using the self policy setting).
For moderators you would add another policy which enables them to edit any topic/post in e.g. a specific section or subtree.
