I have the imsto keycaps (via Feng on eBay) and they fit my Filco Majestouch 2 Ninja perfectly.
The lower profile compared to the Filco OEM ABS keys do allow a bit more gap to show through between the keys to the deck below, but I kind of like the overall effect of this.
But they fit the cherry stems perfectly, as best as I can tell. I also have o-rings fitted on brown switches and another set of the same keycaps on red switches without o-rings, all fitting snugly.
In short: most of the thick PBT keycaps you'll find for sale are all from the same manufacturer (even if from different vendors), but I wouldn't worry about them fitting a Filco!