I am a huge fan of Atmos (and even like DTS:X), but your space being really similar to mine from a size perspective, means that I can give some different insight into the experiences that I had.
The next question would be "how many rows of seats?"
I have 2 rows of seats in my room. I built it for 7."2" from the start, but did do tests with a 5.1 setup before putting the rear surrounds on the wall. I wanted to hear the difference that the money was giving me. I would say that it was noticeably better, especially for the 2nd row.
So I have heard a 5.1 and did a direct comparison to 7.1 back at the time.
Now, I next jumped from 7."2" (it is technically still called 7.1, but with 2 subwoofers) to 7.2.4. So I jumped all in on Atmos about a year and a half ago. I would never go back.
In my opinion, if you have 2 rows of seats, you will greatly enjoy a 7.1 configuration. You will "like" a 5.1.2 setup in your front row, but the 2nd row would suffer as it would effectively be "outside" of the sound *dome* created with the other speakers.
If you have 1 row of seats, then I would definitely go with 5.1.2. Putting them as front heights on the front wall is within the DTS:X guidelines, but well outside the Atmos spec. You would really want to put them on your ceiling.
You could go with in-ceiling M3s, or even some Axiom on-walls and then mount them to the ceiling if you don't want to cut holes, or find that you can't place in-ceiling speakers where you want to due to duct work, joists, etc.
You mention a decorative support beam overhead. I would caution you from putting speakers right overhead. What is happening is you have your speakers up front, and then basically nothing between that and your surrounds (and any overheads that you put in right there). Your system will be "fighting the gap" so to speak and you will still have some separation of audio between the front soundstage, and the surrounds/overheads. I would highly recommend putting the 2 ceiling speakers in front of the listening area (using a measurement that fits within Atmos guidelines). And if you want to be within DTS:X guidelines as well, that IS possible. There is overlap in them. It will push the ceiling speakers closer to your front speakers. This will not only allow for sound to be "pulled" up and above you, but will also help to fill the gap between the front speakers, and the surrounds.
I also highly recommend putting the side surrounds close to a height that is just above head height (so that nobody's head blocks the sound), but leaving them slightly behind the row is fine with Axiom QS speakers. You don't want them directly to the left or right as you would have no part of the speaker actually emitting sound at the listening area.
I've already voiced my comments elsewhere about monopole for Atmos instead of bi/di/quad-pole speakers, but we are trying to work with what you have.
Lastly, for the ceiling speakers, you want something more than "any small speaker firing directly downward." You want some speaker with a bit of "umph" capability to it. While ceiling speakers aren't 100% utilized in movies (yet), when they are needed, the speakers really need to be a decent match for the other speakers in the room or they will fail to be effective. I went from QS8 surrounds in my 7.1 setup to M3 surrounds and M3 overheads and I do notice their capability to provide some deeper frequencies and it is really nice in a movie that actively utilizes the Atmos or DTS:X format.
So my vote(s)... 7.2 if you have 2 rows of seats with plans to add more later OR 5.1.2 if you have 1 row of seats, but put the "overheads" a little less "overhead" to fill the gap between the front and surround speakers.