Recently installed Slackware 13.1 and with it came the latest version of KDE where you can set a window's opacity by right-clicking on its title bar. Is there any way to set ALL windows to a set opacity so that regardless of what window/application I open up, the opacity will be set to that?

Yes it is. Open System Settings > Window Behaviour > Window Rules > New. Change to "Preferences" and tick Active Opactity, selecting "Force" and then entering the desired opacity level, 90.

It worked! This is beyond wonderful!! Thank you! Oh instead of "Window Rules", it would seem to be "Window-Specific" at least on my system...