7mm is pretty thin and will work well, only problem is they'll tear up the hands quicker than a 10mm drill. For the floating technique, I remember learning it ages ago by watching a demonstrational video on youtube. From then it was just practice and it took a few practice sessions to really get the hang of it and then further weeks and even months of practice to be able to generate sufficient downward pressure, for a sufficient length of time to form embers, while only floating. It's a whole new ball-game! What you need to do is practice the technique slowly at first, without applying much downwards pressure, just getting a feel for it - the hand that's moving away from you will be subtly pointed downwards and the hand moving back towards you will be level.
