Ascend Sierra Towers and Horizon matching rear surround and atmos in-ceiling

[avsnoob10]

Hi All,

I own Ascend Sierra Towers and Horizon center since December 2012 all with RAAL. I am moving to a new construction and would like to use my towers and center in a basement home theater. I do not have any surrounds at the moment but considering Luna for side surrounds and possibly rear surrounds.

Attached basement floor plan done in Microsoft Visio from the architect's actual floor/elevation plan for a new construction. Would like to use the space on the far right which is about 21' long and 16' wide for theater. The ceiling height is 9' throughout the basement, except for the 4' wide soffit/framing around duct work which brings the ceiling height down to 8'-0" for that area. I am thinking ideal place for screen would be the far right side wall which is 21' long. Like an idea of an open theater so as people in other parts of the basement can watch the screen from pretty much anywhere however that poses a challenge for rear surrounds speakers as there are no walls to mount them, any ideas, should i make rears in ceiling as well?

How important is it to timbre match the rear surrounds, my side surrounds will be either Luna or Sierra-2 with RAAL? Contractor's low voltage guy suggested i should go with in-ceiling same as Atmos speakers but I like everyone's opinion here. Any recommendation for in-ceiling speakers for Atmos? I know another active user "FirstReflect" posted on one of the Atmos related threads that atmos speakers don't have to match but I am concerned about rear surrounds. Are HTM-200SE good choice for rear surround mounted in the ceiling not flushed to the ceiling but aim them to the seating position? I have seen pictures of couple of owners with ceiling mounted HTM-200SE. Is there a speaker mount I should use to ceiling mount HTM-200SE?

I also posted this on AVSForum about a month ago under dedicated theater build but that was more for space and the layout recommendation than speaker recommendation.