Hey Corsee,
I was there over new years 05-06. Climbed Illiniza Norte, Cayambe, attempted Cotopaxi, and got an hour away from Chimbo - both retreats were because of members of the team having problems with the altitude, although on our Cotopaxi summit day most teams that went up turned back due to avalanche risk up high from recent snowfall.
It really depends on the person as to how they'll do with the altitude. I fortunately had no problems during the whole trip. One guy was vomiting when we got to the hut between the Illinizas at 15,000ft. The next day he was stumbling like drunk up to Illiniza norte, so he headed back to the hut for the day. After that he started taking diamox and climbed like a champ at the next peaks of
Cayambe (18K) and Cotopaxi (got to the 18Ks).
Another guy that was fine at 15K, 16K, and 18K (Cayambe), then had problems going up Cotopaxi and then later also at Chimbo at around 18K... so it really depends. Taking diamox did really seem to help that one guy. One of the other groups we met were on a diamox diet from day one and were encouraged by their "guide" to take them as much as needed.
As for sleeping bags... the huts are warm... very warm with plenty of body heat from everyone around. We stayed at the
hut between the Illinizas (don't sleep above the kitchen-fumes!), the hut at the base of Cotopaxi (15K?), and two huts at Chimborazo,
one at around 15K, and
another in the 16Ks. I had my 15 degree bag and never felt a chill.
The weather in Ecuador is pretty mild and I made it to the top of Cayambe with only a baselayer and a shell (although keep in mind I'm a warm hiker, but it's still at 18,996ft). I would think that even in tents you would be fine with a 20 degree bag. But hey, if you sleep cold then take the colder one.
It's a gorgeous country and the mountains are awesome!
-Doug