JessicaK:
If we have one that we want to add to some day, but inspiration hasn't struck/we haven't had time, is it ok to leave them as "incomplete"? Should we mark them "complete" and then add more if we get to it? (I have one that is two linked stories, I'd like to write more, but maybe it stands on its own?)
Yes, it's OK to leave them as incomplete, all we're doing is alerting the reader that the serial isn't complete and may never be. If at a later stage a further part gets submitted we'll simply remove the "incomplete" status.
Two linked stories sounds more like an ordered or unordered series and this "incomplete" marker doesn't apply to those.