I know installing lots of KDE apps will significantly slow things down because of the additional extensive libraries required, but what about just a few? I'm considering just 4 at the moment: KWrite, K3b, Digikam, and Okular. I assume KWrite and Okular would be low footprint so to speak, but K3b and Digikam might require significant resources (I have 512 MB of memory). If I install these and my system slows down, does un-installing also remove all the libraries or do they remain on the system?
In the meantime, I'd like to hear anyone's opinion on the advisability of installing some KDE software on an LXDE system - will it gunk it up?
It's a common misconception that just adding more packages to an installation "slows it down". Sure, it may take a few extra seconds to boot due to reading the larger directories. But, as krckor and davecs have pointed out, it depends on what
you install. In other words, are the extra packages you install going to start extra system services or daemons? These would be tasks that run continuously in the background while your desktop is running.
As Neal pointed out, a systematic approach to installing the extra applications while noting their dependencies will give you a reference as to what to remove, should you later decide to. Not all of the libraries will always uninstall with the application.
One of my computers is not quite as sparse as AndrzejL's, but it's a Pentium III with 512 MB of RAM. It is running Openbox, which uses only slightly less resources than LXDE. Openbox is the window manager for LXDE. Anyway, as to your K3B question, I've tried all the other CD/DVD burners. For me, they just don't cut it when compared to the features and usability of K3B. So, I have K3B, as well as other KDE apps installed. Although K3B pulls in a few QT libraries and other packages, there is no performance hit on the system for K3B while it's not in use. That goes for a lot of other KDE programs, as well.
Let your own tests and judgement be your guide. And Neal's excellent advice on methodically installing packages.