Well, I've answered my own question, at least for the smaller bag. I can confirm that the smaller FSK bag fits both the 420 and 250 rails. The zipper gets noticeably tight, though not so tight where I'm worried about breaking it.
I have found that you can fit two FSK rails into one bag. You need to move the sliding stop to the bottom and arrange the rails so the stop nests in the cutout of the other rail.