I just grabbed a copy of TPrintPreview and you've done a very nice job with it!
I added a few special features I needed for my application and wanted to offer them to you if you want to include them in the standard component.
The first adds a snap mode to the TThumbnailPreview component so that the width/height of the control snaps to a "nice" size. In other words, it squeezes out unnecessary wasted space from the control based on the current sizes of the thumbnails.
The second exposes the caption of the TThumbnail's. In my case, the pages aren't simple page numbers but need a caption string assigned to them. This gives you a way to assign the label caption of each individual page in the TThumbnailPreview.
Let me know if you'd like the code and how to get it to you. Thanks again for a very nice component set.