I was thinking of doing something like this, too.
Wikipedia has helped me much, and take a look at any Clamp series to back up what I am going to say.
http://en.wikipedia.org/wiki/CLAMPIn the same world:
CCS, xxxHOLIC, Chobits, Angelic Layer, CLAMP Campus Detectives, Duklyon: CLAMP School Defenders, Man of many Faces, Tokyo Babylon, and X/1999
First off, let's talk about what we are told.
In xxxHOLIC, Yuuko says outright that CCS takes place in that world. She also makes a refrence to Subaru from TB, and of course with TB comes X.. The characters from Legal Drug also show up at one point.
Tomoyo's mother appears in the Chobits manga.
Since the Clamp School appears in X, everything connceted to it comes along.
Tsubasa can be counted, since it is a world in which the group visits, but it does not fully take place there.
Other facts.
With the exception of Tokyo Bablyon and X, we don't really know the time in which most of the series take place, am I correct? (TB is set in 1991, and X/1999 is set in, well, 1999) Probably most the other series take place before 1999, with the apocalypse and all going on in X.
Yuuko in chapter 3 of xxxHolic seems to talk of CCS in the past tense, and although she doesn't say Sakura's name, we know Yuuko is speaking of her. Yuuko says she has not met Sakura "and her boyfriend".
From this we can assume that Sakura and Syaoran are older, and that their love has lasted.
My comments.
I like the setup Clamp has going. It's nice to watch one series and know that characters from another are out there somewhere.
Gee, poor cute little Sakura probably lives to see the apocalypse.
That's all I have to say for now. If anyone has other info, don't hold back.