OK, I solved it, even it was a little tricky, but only a little.
It seems to me, that skipmetrosuite can't activate as long you have in any other Programm the same functions activated.
I got back to 8.0 to test it. Disabled in CS both functions to boot into desktop and to deactivate the corners. Also disable Skipmetrosuite. Reboot. Then enable Skipmetrosuite, with all functions. Reboot.
At this point the Skipmetrosuite worked, but it's a Little slower than CS. For a Moment you can see the start Screen and when the Desktop is showing up it Needs half a second (or so) to disable the Corners. But now it was possible to reanable both functions in CS also and it works now together with Modernmix without Problem. And because of the enabled CS functions, I don't see the start Screen when booting and the Corners are disabled as soon as the Desktop Shows up.
Yeah!
So I think it should be possible, too with 8.1 if you disable also first the same Features that come with 8.1 (booting to Desktop and top Corners)
(Sorry for capital letters, it's the shitty Internet Explorer. I didn't install yet another because I wanted to test this.)
Unfortunately there's a problem with my Wlan stick so I will stay at 8.0 first.