I asked Gregor about what would be the prerequisites for beginning maltese training on rings for someone who is not planning on obtaining the planche here is his answer for those interested:
"All those *referring to my question about having bl, fl, and ironcross for maltese work* and not in their easiest variant (MU to BL, kip to IC or azarian and so on) including the planche training. No matter if your goal is maltese, you need to train planche (even though if you'll never hold it). Planche is foundamental training for maltese or at least at the begining. When you'll be strong enough to hold good solid spotted maltese then much more focus on maltese training.
Don't ever forget auxiliary exercises and prehab.
This is just a quick replay for more detaild information
i would need more insight of your strength and training, what is hard over the net."
If anyone else has something to add, feel free.