Will Maggie 3.6 or 20.1's work in a 121/2 X 21 Rm

The short answer is yes, either speaker could work and sound quite good in a room that size. You might want to experiment with the ribbon tweeters being either on the outside or inside to get the best balance between focus/details and size depth/width of the soundstage. I have heard many Maggies sound terrific in similiar sized rooms. Hope this helps.
I had Maggie 1.6s in my 13 x 22 room and always felt they had too much output from the QR midrange driver. I used ficus trees to help dampen the reflections and it was pretty good but I think 15 feet wide would have been better.
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.