They are all great, and they would all work in your room. The 3.6's are a great value currently, and will likely be replaced by a new model soon. What's your gut feeling after listening to them?
I own Thiel 22's. My room is 15x17 and with 22's, I've had to position my speakers almost four feet into the room away from the front wall and almost three feet in from the side walls. With that in mind, I feel that the 3.6's would be a little on the large side for your room. If you are considering the 3.6's, I'm assuming your budget would allow you to add good musical sub to the 1.6's or upgrading to the 2.4's which is a huge improvement over the 2.3's.
When I was speaker shopping in early 1995, I was considering the 2.2's and the 3.6's, which at that time sold for $2750 and $3950 respectively. My wife and I took some CDs to the Thiel dealer in Tustin, CA, the Digital Ear, and a very accommodating salesman gave us over two uninterrupted hours to listen to the 3.6's driven by Krell and Audio Research electronics in a listening room that was designed with Thiel's advice. The 3.6's were just awesome, and my wife encouraged me to spend the extra money and get them. Their greater volume and resulting fuller sound, deeper bass and spatial imaging are very helpful on the classical orchestral recordings that I listen to. I bought them and I haven't regretted it. My listening room is about the same size as yours, and every improvement I have made to my system since then has been instantly audible and sonically rewarding.
The 3.6's are wonderful speakers that you won't outgrow for a long time. You can find a used pair for less than $2K, which is an incredible bargain. As you've mentioned, they do need room to breathe. (See the description of my system elsewhere on AudiogoN.) They also sound their best when driven by quality electronics. I urge you to consider very seriously whether your room can accommodate them, and proceed accordingly. Good luck!
I went through a very similar decision-making process. After auditioning 1.6's, 2.4's, and 3.6's, I preferred the CS3.6's (yes, even over the CS2.4's!), although the CS1.6's with the SW1 sub was pretty darn nice sounding, too. What I like about the 3.6's is that they are a bit more full sounding, and they can convey a sense of "weight" that the 2.4's couldn't match. Yes, the 2.4's (and 1.6's) are even more focused and precise in the upper mids, but to me, it didn't sound as musical as the 3.6's (which admittedly sound a bit veiled by comparison). But playing some big orchestral works on the 2.4's, and then hearing the same piece on the 3.6's really convinced me. Plus, with the 2.4's or the 1.6's, I would definitely need a subwoofer to get a level of low end response that is satisfying to me, whereas the 3.6's don't necessarily require a sub, with the right amp. In the end, I bought the CS3.6's, and have been very happy with them, even though my room is not very large, either.
Speaking of amps, I also own a GFA-555II. Two of them, in fact. I had been driving the CS3.6's with each 555II in bridged mono, which is a boatload of watts, but which has the potential to present the 555II's with a difficult load to drive. However, the Adcoms run cool and clean with no problems. Yes, this setup sounds very good. I am using a Rotel RSP-1066, which isn't top notch, but I feel is quite good for the money. With all that being said, though, I am presently considering upgrading my amps, and most certainly will do so. I am hoping to A/B my Adcoms directly against Krell, McCormack and Theta, but Pass, Bryston and Classe' are also on my "short" list. The Adcom GFA-555II is a fantastic amp for the money, but I do think that Thiels in general, and the CS3.6's in particular, will reward efforts at upgrading the rest of your system. They are really quite capable speakers.
Being a Thiel fan for years, I would be inclined to think that you would be satisfied with either the 1.6, 2.3 (although 2.4 is a significant improvement) or 3.6. With the 1.6 or 2.3/4, though, a sub is necessary, IMHO. And with any of their speakers, an upgrade from Adcom components would yield definite sonic improvements, though in the short run, I think that they would sound fine, and by the time that you save up for an upgrade, you will probably be in a good position to appreciate the sonic improvements.
Good luck, Tom.