Not quite done! I did this, and I had problems with tech purchases disappearing. I think this is because Interstellar's tree.cfg is outdated. When the game loads, the outdated tree.cfg and squad configs try to put the same part in different nodes (e.g. squad configs put the turbojet in supersonic flight, KSPI's tree puts it in high altitude flight, if you buy the tech, its purchase isn't saved in its proper node, and when the game loads again it thinks you never bought the turbojet). The way I fixed it is I went through the tree.cfg and removed all the stock parts. They don't really need to be there, since their tech node is already defined in the part's .cfg. Issues disappeared.