TL;DR If I set a fixed window size, do i still need to define the LayOut with the "Layout in a grid" or "Layout horizontally"? Or can i leave it like that for small tools....
If you plan for fixed widget/window size, there is no need to have Layout in widget (QGridLayout/QHorizontalLayout etc). you can just manually position each of the child-widgets inside the parent-widget (from Qt Designer)