Hi problem,
I've been researching LL for the past 2 months, like you I'm new to it. Here is what I suggest. Since you are 21, there is a chance you might still be growing so I might hold off a bit. Also, those doctors you listed might not be reputable. I can't remember off the top of my head, but there have been many surgeons from Turkey and India that have left people on this forum with complications, some have been left crippled for life. If you really want to do this, I would recommend saving up for a reputable doctor in the states (Paley, Mahboubian, Rozbruch, Debiparshad) or maybe Dr. Donghoon Lee or Dr. Giotikas, on the cheaper end. I would also go to the YouTube channel Cyborg 4 Life and watch every video on there, it will answer your questions and much more. To answer your questions now: 1) femur is usually recommmended over tibias because there are less of a risk for complications, 2) The max with a precise/stryde nail would be 8 cm, but 5 cm if you want a good shot at retaining your athletic performance. You're pretty young, so I suggest doing your research for about a year and save up for a good doctor that will give you the best shot at recovery. Good luck.
-brondo