Some do IMO, and some don’t. For example, I think Technique, Step Up and the original High Voltage don’t really fit because they’re very hip hop driven, more so than any of the stuff on Hybrid Theory. You could argue that CFTI is too but it’s a bit different. Now, the reworked version of High Voltage that was cut from the album with the CFTI transition would have worked because it fit the sound better. It’s more the beat of the old one and the style of it that wouldn’t have fit IMO.
Carousel, And One and Part Of Me are the big rock songs of the release. And One I think would fit the best, especially the version they were playing live during the HT shows, because the HTEP version has the rap outro and that wouldn’t fit the style of the album. Carousel would have worked if they kept it sounding more like the HTEP version, the versions after it changed the whole vibe and atmosphere of the song drastically, it made it sound almost poppy in some ways, while the HTEP version is extremely dark and menacing sounding, very hard vocals in the chorus that are amazing, and would fit with stuff like the hard vocals on the final Crawling, etc. Part Of Me I think would fit well, has the vibe of the album and I think a cross between the more finished version with the screaming of the ‘’cut myself free’’ vocals and maybe the style of the chorus on the HTEP version would work good. Of course the hidden track would be cut, as epic as it is, it would not fit.
As for other demos from the era, I think Slip would have fit on the album if they had tuned it up more and re-recorded vocals. The rest I can’t really see fitting, even though there are great songs like So Far Away and She Couldn’t, they were too ahead of their time to make the album IMO.
And one last thing, n00bs always say My December, but that song was written months after the album was finished, so it makes no sense, and it doesn’t fit with the style of the album anyways as epic as it is. It’s more of a re-release bonus track if anything.