Never tried growing socortranum ...have heard they are extremely slow growers and very temperamental ... but here is my 2 cents worth
to *start the seeds* .... I see nothing wrong with using the store bought cactus mix with extra vermiculite
but once over an inch tall or so ... I would use a rocky mix, with large grain sand ....and very little organics
These socortranum plants initially came from Socotra an Island out in the Gulf of Aden and Arabian Sea - between Somalia and Yemen ... possibly one of the hottest, driest places on the planet ....