So far it looks like even in recent decades, the Scott Internationals remain somewhat selective. It would be useful to find out how selective, though, in other words to compare a number of countries over the last four decades or so to see approximately what percentage of stamps are included in the Internationals.
I'm not asking because I'm deciding which album to use. I'm just curious. I collect using a few Scott Specialty (and many Davo) country albums for those countries I collect in depth as well as Scott Internationals for other countries (plus duplicates of the in-depth countries). So, I'm not too bothered by whether Scott includes all, most or just many stamps. I'd just like to know if there's a general consensus on how comprehensive the Internationals are for recent decades.
As for adding Steiner pages inside page protectors, I'd be more inclined to get pages I needed printed onto actual Scott Int'l sized paper (it can be done) -- if I decided to supplement the International album, which I might do for some countries.
On the different subject of the Scott catalogues (and albums) separating regular/commemorative stamps from semi-postals and airmails, I've always found that pointless -- and I live on the "correct" side of the pond!

Scott's idea beginning a century or more ago of separating stamps by type may have made sense at the time (when people collected postage dues, officials, envelopes, and many other types because there were so few "real" stamps). But it doesn't make much sense today. I much prefer the Stanley Gibbons, Michel, and others' approach which is to combine them, including them by year and not by type. Interestingly, Japanese catalogues do as Scott does, separating stamps by types. Very confusing and a bit strange, I think. In fact, in all my Scott albums I take the semi and air mail (and any other) pages and insert them as chronologically as I can so stamps from, say, the 1920s are with others from that same era. It's not always easy to do that, but it just makes a lot more sense to me.
In fact, Davo albums also separate stamps by type now that I think of it -- so also with those albums, I often put the semi-postal and airmail (and souvenir sheet, etc.) pages with other pages from that year or years. Stamps of the same era tend to reflect the same topics and issues and have similar styles -- my main reason for not wanting them separated into many different sections of an album.