Yeah, I think that's a good way to describe ANIMORPHS, although I think it's far more character based than X-Files ever was.
I'm actually still figuring out the same thing, re: not contradicting what came before
I'm relying on my editor to keep me on the right path. However, we all know the major "plot beats" for each book so we do know where the characters and the plot are at by the time we write the outline. And we get to read the other manuscripts as we go and the new outlines as they're written. So, for instance, I'll get to read the first 4 books before I get started, so I'll know the series voice and the characters and the world pretty well before I dive into writing the book. In fact, it's funny how well I feel I know the characters even just based on reading plot outlines.
However, it's those three books before mine, the ones that are still in various stages of completion, that may cause problems. For instance, I don't even know what happens in book 6 yet because there's no outline. I have to go by plot beats only.
Basically, I get to do the best with what I know, and if I make any mistakes, well, that's what I'll get to fix in the rewrites. Either that or the editors will fix them for me.
REMNANTS was a little different in that the outlines were extremely detailed. Since the same person who wrote the outline is also writing the book itself with DRAGONLANCE, the outlines can be relatively vague. The REMNANTS outlines, however, detailed everything, so I had a really good idea what I was doing when I outlined book 7. (Just to give an example -- my DRAGONLANCE 8 outline is 7 single-spaced pages while my REMNANTS 7 outline was 20 single-spaced pages, although that was long even for that series.) Even then, it wasn't until I read all the manuscripts available that everything clicked -- lots of little details to pick up on. It's a tricky thing, but it's definitely doable. I remember one time reading an e-mail where the author of one REMNANTS book talked about rewriting the beginning of her book because one of the other authors specified that the water the characters were in at the end of her book was cold, whereas the other author had went with warm water. I guess you just have to look at the series as a constantly evolving story instead of everything that you write being set in stone. You also have to be incredibly willing to work off of the ideas of others, otherwise you'll be incredibly frustrated.
Speaking of that, I recently found it's important to look at what's coming next as well. Now, we're all just making this up as we go along -- it was the same with both series, which is why it's so important to leave things open when you're creating a book in a series -- so there's nothing set for future books that must happen. But a friend who may potentially do the 9th book in DRAGONLANCE has been asking for my opinions and whatnot on his own plot sketch. He ended up writing in something that embellished upon the ending of my book that made me entirely rethink how I want to play that scene when I eventually write it -- for the better, mind you. So while he'd be responsible for not contradicting anything I write, I can help out -- and make for a more intriguing and solid story -- by looking ahead and foreshadowing what he will write. It's very much a team effort.
Re: HARRY POTTER, I'm not sure whether to consider it a series or not. I mean, we don't consider the CHRONICLES OF NARNIA a series, though both will have the same number of books in the end. And then there are all those series that we DO consider a series, even though they never made it past six books. Of course, there's also A SERIES OF UNFORTUNATE EVENTS, which we consider a series despite that, like HP, it has a set number of books -- 13. I really don't have a way to clarify what, exactly, makes a string of books a series. Anyone?