As far as I can tell, the Extra Planetary Launchpads version I have (v5.2.94) does not come with Interstellar Fuel Switch, at least not inside of its plugin folder, and I don't seem to have IFS as a stand-alone mod in GameData.
Therefore, I will gladly keep both parts of that mod inside the otherwise empty-to-be SimpleConstruction/plugins folder and gratefully use the wider and shorter ore tank for the metal and rocket parts.