Firstly, thanks for your help! I appreciate.
I plan, that e.g. the square can beat certain some enemies and the circle don't. That's why I thought global variables and different sprites were the way to go. Is there a way to do that, if I only have one sprite with different animations?
But even more important: I didn't find a way to change the kind of the general collision mask from "bounding bx" to "circle". So, when I change to the circle, on diagonal platforms it shakes like hell. If I choose "circle" from the beginning it's vice versa. The square and triangle are shaking. I thought, with different sprites and a placeholder, I'd avoid that. And that's the point where my problems began. I think, a command to change the general collision-type would be one solution. But thats not possible, right?
Thanks for the every tick-hint.