Some question about gauss's law in Catesian Coordinate

[baby_1]

Hello
I have some questions to understand much more better the Gauss's law in Cartesian coordinate.
1-when can we use Gauss's law and it's integral to solve a question easier in Cartesian coordinate?

2-Is it difference to use a cylindrical or cube shape for a plane that disturbed some charges(P0) on it?
3-If we want to use Gauss's law for a plane we should use symmetric Gaussian shape or not?(see these two pictures)

or

4-Should we define electric filed direction ? if yes , how should define electric filed direction?
(some examples a,b) it's direction is it related where we want to define electric filed value?
(a)

(b)

 

[baby_1]

6-if there is no problem could you tell me a good reference with a lot of example about Gauss's law in Cartesian coordinate?