I'll figure out the total configuration later, but my last question is at how much of an angle can I place the center speaker? Is there a problem if I exceed say, 20 degrees? Thanks.
With your centre speaker that high above the listening position I would seriously consider one of Axiom's horizontal designs vp100/150 instead. You will benefit way more from their superior vertical off axis dispersion capabilities than you will trying to timbre match your front LCR.
In your case, even with three identical front LCR speakers you won't get a proper timbral match, nor will you get a seamless front soundstage simply due to the extreme location of the centre relative to your mains. Unless you have seating way way off to the side, you should be more concerned with the vertical off axis performance of the centre speaker.
Trying to timbre match using identical front LCR is fine, but that is assuming they are all on the same plane and with placed in identical / similar surroundings (ie. distances from boundaries, behind or in front of screen, etc.). Non-ideal placement can level the playing field when comparing vertical vs. horizontal centre speakers.
