Our product manager has indeed listed the Bosch Professional GKS 12V-26 as compatible.

Yes, the GKS 18-57 is compatible with the FSN guide rails and I have used this combination successfully several times ;-) (see also https://www.bosch-professional.com/de/de/products/gks-18v-57-g-06016A2101 )

Compatible is (only) the 2-piece guide rail with 35cm each made of plastic incl. screw clamps, Bosch order number: 2609255732, EAN code: 3165140547031