I recently downloaded Pets, Island Paradise, and Into the Future, and I'm used to having to weed out incompatible content after installing EP, but since installing those, I can't install new CC. At first I thought it was a couple of problem files (which when I removed, the game ran smoothly), but the files I removed weren't files that couldn't be compatible with EP's, they were a couple of CAS hairs (specifically, Fanaskher's Gaga Venus, Gaga Grammy, Miles Away, and Bittersweet retextures). I've never had problems with CC hair being incompatible. And not only that, but when I tried installing different CC after removing those files, the game wouldn't load. I've tried removing the mods folder and clearing the 5 cache files, starting up the game, quitting it, and adding the mods folder back. Nothing seems to be fixing this.

I've heard some versions of the game require the EPs to be installed in the order they were released, whether you just use a couple or all of them. If that is the case, I would suggest uninstalling all EPs then reinstalling them in that order.

I've never had CC refuse to install, though several times it did not appear in build/buy for days or even weeks after it was installed. In terms of random glitches this has to be the screwiest game I've ever dealt with.