Hi Wen,
Yes, I've often started the same as you with and Introduction Forum where "Each person posts one discussion".But I've found is that whatever type of forum you use first, students will assume that all the others will be the same. So later when I want everybody to have a "conversation" and reply to each other in discussing a subject, they ignore each others' contributions and start a new discussion.
But in the last few weeks I've stopped using the standard Moodle forums because of all the advantages of the Open University's ForumNG developed by Sam Marshall. There are a few disadvantages because it's not fully integrated into Moodle (for example, the forum search engine doesn't find ForumNG posts) but these are minor compared to the all the plusses.
Cheers,
Glenys