Yes, it is Ardupilot page, but includes some good summaries (e.g. ground station softwares, VTOL features).
Yes, I have read this page, so I understand, how to build the mixer file. I made 2 mixer files, what I have to fine tune of course (my quad is not finished, and the Pixhawk not arrived yet). I have built a test bench, where the quad is fixed to the ground, but can rotate in all 3 directions, and I can fix the axles one by one. It is not the same as in the air, because the turning point is not in the weight point of the quad, but it is good for the first test. I have tried it already with the smaller electric version and KK2.1.
I also plan to make a bigger bench, where the quad will be fixed by rubber bends, so it will be very close to the real flying. However the aircraft mode test is a question mark yet.
Then, what is the next step?