Well, by and large, the apps are independent of the IDE. KDE apps require KDE libs and Gnome apps require Gnome libs, but they don't require the IDE, as such. Some stuff like certain configuration tools or what have you might not work, and some act funny or need tweaking, but they most all work with any wm. And, as far as general apps like mozilla, yeah, those all work. But those apps will still be on the system whether you use them or not unless you delete them. You'll save memory and things will generally be faster - and simpler - with a wm.
I was a blackbox freak until I tried fluxbox which is 99% like blackbox and I like the other 1%.
Either of those are my favorite though, for something a little heavier, there's stuff like icewm and windowmaker.
Oh - and you install them just like any other app.