This kind of question is quite general and there is no well-defined answer to it. The search of global minima of interface is quite challenging because of large numbers of degree of freedom in the energy landscape. The simply use of geometry optimization would fail or take very long simulation time. In literature some groups tried to apply some evolution algorithms to search the interface structures. It seems to look promising.
"how do I decide the x,y,z shifts (under 'Shift Surfaces')?"
You may choose an initial values depending on the covalent or ionic radii of atoms in the interface regions.
" Is the 'geometryoptimization' strongly depend on the initial structure?"
Of course, it does.