not sure about derviatives, file types, and export for lightroom

Hello:
I am a new lightroom user and new to the DAM process. I am confused about something that is probably fundamental to this process. I have two internal drives on my computer (one working drive, and one for a photo archive for originals and derviatives), as well as external backup and burning to dvd. I will be using lightroom for both image adjustment and catalog management (will not be using photoshop at this time). Also, plan to convert to DNG on ingestion for raw.

Here goes: once I ingest from a card and have added bulk metadata, rated, and performed some basic image adjustment these images will be moved to the archive as originals.

Now, if I perform further work on any of these images (i.e., addl metadata, addl image work, etc.) would these images then be considered deriviatives? If yes, would I then go to export in lightroom and give these images a new name (e.g., master file, master.dng) before sending, for instance, to working derviative folder?

Also, if I am sending these images to archive as derviatives, but not sending the images out for email or web do I need to change the dng to another file type (e.g., tif ,etc.)?

Lastly, I have some old jpegs on media cards that I will be using to go through this process. Is there anything special that I need to do when handling these jpegs versus all future pics shot as raw?

I have read the DAM book as well as other online resources, but the whole process can be quite confusing for the beginner. Please respond anytime.
Thank you
Joe Giacalone