They don't mix. I recently downloaded Monolingual since I have heard about it, and recently found out it was Universal. So I opened it on my Intel iMac, deleted a bunch of languages, which was cool, and then I saw the Architectures tab. Bad idea. I deleted all PPC based execution code. I forgot Photoshop was PPC. The weird thing is, Activity Monitor, Installer and a couple other utilities didn't work. And since I had to use Installer to reinstall the previously mentioned dead Photoshop, I had to reinstall OS X. Not fun. The moral of the story? Don't play with guns.