Thanks, amazing thread and OP!
I love grafting cacti a lot.. i use P. spathulata, P. aquosa, Selenicereus grandiflorus and now also some Hylocereus sp. as rootstocks.
What are you guys' opinions and experiences regarding those different species? And do you think some are better for certain scion species and other rootstocks for other scions?
I mean mostly for seedling stage here by the way. For bigger ones I definitely recommend trichs, myrtle or other columnars but i don't currently have the luxury of having a lot of those handy because I have to grow exclusively indoors without all that much power.