What is the way to configure Smoothie, that the actuators don’t travel farther then I told in e.g. “alpha_max”?
That feature ( called “soft endstops” ) does not exist yet. It is more complicated to implement cleanly than one might expect.
https://docs.google.com/document/d/1U6nzx1boqF-J2GGPWF4yIaaVib0JNodVWSKBfwiyp_M/edit#
Thx for the fast response again 
So to be safe I better add more limit switches for now?
Well it’s not like on most machines you are going to break something if you go over limits. But yes, limit switches are the solution if you are worried.
I never had this happen during printing. It only happened during calibration-orgies…
True, but I hate the noise of motors missing steps. And it kind of tickles my “perfectionism” - must be a German thing 
@Thomas_Balu_Walter What do you say about not having any endstops at all at X&Y? 
@Rene_Jurack I’m seriously thinking about paypaling you 10 bucks, so you can order a few.