If the book is going to be saddle stitch bound, you need multiples of 4 pages. If perfect bound, comb bound, spiral bound, etc, you need multiples of 2. That's why you see spreads and why the pages shift when you add one. The first suggestion, keeping spreads together, was proper and the easiest solution to your problem is to add 2 pages. Then think up something to put on the second new page.
If you have different sizied text frames and they are not set up to flow text from one page to the next, it is impossible to move the text from the bigger one to the smaller and have the text fit. The only answer is to go back and thread the frames since, I'm assuming, you didn't use a master text frame on the original document setup.
Using OSX 10.3.9 on a G4