Will CC wreck my game?

Other people have covered most of the bases, I'll just add that if you do end up having problems in your game because of bad cc or because a patch ends up borking something, you'll need to do what's called the 50/50 method to fix it. There's a pretty good explanation about how to handle problems here:

http://modthesims.info/wiki.php?title=Game_Help:Finding_Problem_Custom_Content