I was looking around for some of my pencil tins (Dan and I are comparing what we already have before we avoid bidding against each other to acquire a couple more) and lo-and-behold... I found my Pelikan mini ink cartridges that came with the Kickstarter Tiny fountain pens.
And even better than finding more ink cartridges (because there's not enough other ink around for our pens), they fit the Sailor 21!
Voila, ability to write with an amazingly fine nibbed fountain pen without treating it like a dip pen.
So, a note for the compatibility list: Pelikan mini cartridges are slim enough and just short enough to work in the vintage Sailor 21 pens.
Still planning to get some of the Sailor mini cartridges soon. I want to try out their permanent pigment ink and see how that differs from the inks I'm using in other pens.