I have my 3.6's set up in a dedicated, treated room, 17 X 23 x 10. Speakers are oriented on the long wall, 4 feet out from the rear, about 8 feet from the listening position, 7 feet apart inside edge to inside edge, with the tweeters on the outside, with a slight toe in. There is about 3.5 feet behind the listening position as well.
If I do say so myself, I am getting wonderful results. Plenty of bass, and enough distance between the listener and the panels to give the sound room to gel.
The long wall orientation is a bit unorthadox, but it was based on measurements and models done by Rives Audio.
My initial impression is that while the 3.6 might work in your situation, provided you can experiment with equipment positions, the 20.1 may over-pressurize your room, given the requirement of the loudspeakers amplification needs.
The above statement comes with a qualification: small chamber works and small jazz ensemble works may be fine, but you might be pushing the envelope (literally), with large orchestral works and high-power rock with the 20.1.