Ah okay now I see the problem. Unfortunately I didnt use angular fxLayout, but if its basic behavior is as flex box’s, I can give it a try.
To answer your questions:
Yes I think so, if you dont wanna set something specific, try to use percentages. The renderer needs to know its size, you cannot say stretch to fit its containing div.
this example does have a resizable/responsive 2/3 to 1/3 layout with flexbox:
Dont know how to make a gap between with flex tho.
Maybe someone else can help here
Hope this helps