If I update an RSS feed, and I already have that feed on my Palm, will the new messages be "added to" the existing document on my Palm or will the file be overwritten? I'm guessing it's the latter. If I change the name of the file in Plucker before updating will this prevent it from being overwritten?

I'm guessing this is one of the problems that has led you to making your own viewer.

Now it appears that if a new message exists, it will just download the complete list of messages. It doesn't work like a news aggregator which just downloads the messages you haven't already received.

I believe that that is the way it is designed to work currently. As I understand it, it currently checks to see if the page has been updated since the last download. If so, the whole thing gets downloaded.