Archival clips that are 15fps? Where do these come from? No tape runs at 15fps? Old films (talking REALLY old...silent era) ran at 18fps...were telecined to tape at 29.97fps.
The only clips that I know that run at 15fps are ones encoded for the web. So are those the ONLY ones you have access to? If I were in your shoes, I'd import them into a 29.97 project...as that is a multiple of 15. And then bring the bin of footage into the 23.98 project.