And what I am about to say may get flames.. but I will say this anyway. Starting out on a smaller bike like the KTM RC390 is definitely the right choice as a beginner rider. You will learn the characteristics of how it even feels to be on a bike. On a smaller bike, at least on the RC390, everything feels softer, lighter, and more forgiving. But now coming on to a bigger bike like the RS660, the littliest twist of your wrist will send you to stratosphere, not to mention the accerlation of how quick it gets you up to speed.
Not to mention, a big thing, I think especially applies nowadays on small bikes vs. big bikes is that the rider aids. Now, I know the RS660 is rider aids galore, but generally, a lot of bigger bikes has a lot of rider aids like QuickShifts/Autoblipper. And as a beginner, you may rely on these rider aids and miss the opportunity to learn the magic in shifting the bike and maneuver the bike at slow speeds like U-turns and stuff. Essentially, giving up the opportuunity to learn about good clutch work and stuff. Ultimately, riding is not just going straight, its cornering, U-turns, stop and go traffic as well. Also, learning to shift and good clutch work will allow us to enjoy other bikes. Another plus on starting out on a smaller bike vs, going straight to a big bike.
Coming back to it, if I had gotten THIS bike as my beginner bike, I think the learning curve will be much bigger. I think eventually you will manage, but how many more scraps and scratches will result before you get there. In the end, I think it was the best advice the Internet Gods can give to beginner riders to start on a smaller bike rather than a big bike.
I like the way you think.
No rider aids are a great way to develop skills, but also doing it in a more extreme, but controlled environment also builds skills fast.
Easiest way is to go to a parking lot and practice U-turns, emergency braking with all your rider aids off. Unfortunately, part of the Euro 5 spec says you can't turn front ABS off, so if it's a really new bike, this may not be possible. Buy a cheap set of cones from Walmart and look up cone drills on the Internet - offset slaloms, cloverleafs, time yourself doing the GP8 gymkhana drill, etc.
An even better way is to take a course like Trail Tours in the dirt, or Fast or Racer5 on the track. Their bikes have zero rider aids and will build skills at an exponential rate, faster than anything you would ever learn on the street or even a parking lot.