Regarding castille soap (= pure olive soap), you can try with it for sure. However, be aware that this surfactant is stronger than coco-glucoside so it may dry out your scalp more than the original recipe. The great thing is that DIY haircare products can be adjusted based on your needs so you can also reduce the amount of surfactant if you see that castille soap is too strong for you. If you want mild alternatives that will have more similar effect than coco glucoside we recommend using Decyl glucoside or Lauryl glucoside.