When you install the center channel in-wall, you are introducing a number of diffraction effects into the system that are not tamed if the center was not designed to be in-wall mounted.
In my opinion, the best (home) theaters did not mix standing with in-wall speakers.
If you are interested in running a theater with your 2.0 setup, select some nice in-walls for hiding out of the way of your 2.0 setup - and, as Theo suggested - keep them lined up: the driver arrays often sound most engrossing when they are kept in the same plane.
There are many options; let us know your progress!
In my opinion, the best (home) theaters did not mix standing with in-wall speakers.
If you are interested in running a theater with your 2.0 setup, select some nice in-walls for hiding out of the way of your 2.0 setup - and, as Theo suggested - keep them lined up: the driver arrays often sound most engrossing when they are kept in the same plane.
There are many options; let us know your progress!