Go through the topic toppleover posted - it is an epic but it has a lot of good information. I also agree with Kamikuza, in that it is sufficiently different from normal directional boards that experience on them won't automatically make you more likely to succeed on the foil.
I've ridden most things behind a kite (wakeboards, twin tips, mutants, surfboards, race boards) and the foils need a completely different style and balance to all of them. The race boards would probably be the closest, but still very different to those (race boards are completely different from normal directionals too).
I'm still an absolute beginner on the foil myself and only had my 3rd session today... All 3 sessions have been in around 10 knots on a 12m 2014 Naish Park. When the wind is dropping below 10 knots, getting out of the water can be a challenge, but once you get up on the wing it just takes off and you go upwind like mad. I haven't been out in 15 knots yet, but I'm guessing that you would be close to wanting to switch down to the 9m by then.
What I've done to try and speed up the learning curve is to use shorter masts... For my first two sessions, I had a 15" mast. This allowed me to get a feel for the balance points without having dozens of massive stacks - if I screwed up and put my weight back too far, the foil would come up and breach, but it would only come out a foot and I would just fall back onto the board and keep riding. Falling 3 foot from a full-length mast breach is very different than falling from 1 foot... I spent around 2.5 to 3 hours riding on the short mast until I could keep up on the foil consistently.
Today, I put the 22" mast on and had a blast. Still plenty of stacks, but I could also still recover from most of the breaches. I had plenty of nice, long, silent rides completely on the foil. I was also able to do all this in slightly deeper than waist deep water (because of the short mast), which makes relaunching the kite in those conditions a lot easier (I dropped it twice)... The majority of the stacks I had today were not from breaching, but were coming off the front of the board because of the way you need to have so much of your weight forward rather than back on all the other boards.
I detailed this a bit more on a post on this page:
www.seabreeze.com.au/forums/Kitesurfing/General/Entry-level-foils-boards-for-beginners/?page=18#1865271 I know the short mast thing will not be for everyone, will limit your choice of foil (basically to the Aluminium ones), and will cost more (buying an extra mast), but I just thought I would mention it because it's not something I had seen discussed much, but it makes a lot of sense when you see it in action and hear the stories from the majority of guys that learnt 'the hard way'...
www.seabreeze.com.au/forums/Kitesurfing/General/Entry-level-foils-boards-for-beginners/?page=18#1865271