To addressing concerns more directly about your "Big Blue" Scott International album, eight (8) binders means you have a lot of pages. But it's perfectly easy to spread your pages out into a ninth or even a tenth binder if you need to add a lot more pages for your extra stamps. That seems like the simplest solution for too many stamps with too few spaces to mount them in.
The problem you're having is because the (blue) Scott International is an abbreviated version of the original (brown) Scott International album. The "Blue" version omits many less common stamps Scott thought most collectors would not have.
Originally, Scott International pages included all stamps issued by every stamp-issuing country. In the late 19th Century, you could do that in one volume. They're called the "Browns" because of the color of their covers which distinguishes them from the later "Blue" Scott International album we have today. You can sometimes still find Brown International albums on
ebay (not usually in very good shape, though).
Since the album originally was hardbound with sewn bindings, you couldn't add pages for new issues. So Scott added new volumes every once in awhile. Each volume began over again with the "A" countries for that era, which is probably why the current Scott International volumes also start over in each volume with the letter "A". Eventually, the first brown album was labeled "Nineteenth Century" when a new "Twentieth Century" second volume was brought out. Later, a third volume was published for the 1920s, so the second volume was labeled 1901-20 (I think those dates are right). A fourth volume was published for the 1930s before the idea of hardbound albums ran out of steam.
At some point in the late 1930s, Scott also started publishing a more abridged version of the Brown album for the general collector and for beginners. It had only spaces for more common stamps. That compared to the Brown albums which had spaces for every stamp. The abridged album was called the Scott International "Junior" album. It covered common stamps from 1840-1940. This is the blue Scott International we have today. "Junior" was eventually dropped from the title.
The Blue album leaves out stamps Scott thought the average collector would not be likely to find. But, many "hard to find" stamps from that era have turned out to be fairly available, after all – and some stamps Scott omitted which are genuinely hard to find can sometimes be found, anyway. But the blue album has no spaces for these stamps. A set of stamps in the "Junior" or "Blue" album is likely to be represented only by the more common low values and omit the harder to find high values. If you buy the whole set, there's no space for the high values. So, collectors with more comprehensive collections get frustrated with the more abridged Blue albums we have today.
By the 1940s, 1950s and later, as Scott published new Blue International volumes for each new decade, it was apparent that, unlike in the earlier years, most stamps were now commonly available. So Scott just included more and more stamps in each new volume without ever changing the first 1840-1940 volume. So, after 1940 (or so) Blue album pages are fairly comprehensive. It's the first volume that covers 1840-1940 that is missing many spaces for stamps.
When Scott stopped publishing the Brown albums, they weren't about to throw away all their old Brown International pages. Instead, those pages became the green Scott Specialty albums. They're much more complete for the 1840-1940 period than the Blue pages, but they're pretty similar (if not identical) after that.
A few years ago, Subway Stamp Shop made a deal with Scott that Subway would reprint the entire older set of 1840-1940 Brown International pages in loose-leaf format on single-sided paper as a kind of luxury album. They call them "Vintage Reproduction" pages.
So if you want completeness you can either buy Scott Specialty albums for all the countries you want, or you can buy Subway's reprint of the "Brown" pages and add them to your Scott International album. You'll need a bunch more binders to hold them.
A less expensive option is to use blank International pages, as you're doing. To keep your stamps as much in chronological order as possible, I'd put blank pages throughout the early years of each country. Eventually, you'll need a few thousand blank pages. They're "about" 50 cents a page the last time I looked, so also not a cheap option. Or you could have a local print shop make you a few thousand pages in a similar color in that same size. Have them hole punch the pages for you.
As for page headings, a print shop can do that for you, or use your own labels. I might handwrite them, myself, since I like albums where the collector has given their collection some personalization.
With blank pages, I'd organize extra stamps by year, not by type of stamp. I've never understood Scott's method of separating stamps by type so that stamps from the same era, with the same style and subject matter, are kept pages apart. I prefer keeping air mails, semi-postals, and other stamps together since they were issued together. This is the approach used in most European albums. It's your collection, though, so arrange them as you prefer. That's none of my business!
If your pre-printed pages have labels, then label the new stamps. If not, then don't. By labeling, I mean the date issued and perhaps a few words about the subject.
Hope you enjoyed the history lesson, and I hope this helps a little with your collection.