The KDE font manager takes care that you only work with the fonts you really need. You can decide whether or not to use the set of fonts in your X11 font directory. You can start the font manager using the Application Starter. The font manager can be found in the System folder. When you start it, you will see the list of X11 fonts available in the left window and the fonts used by kde in the right window. If you wish to add or remove the ability to use these fonts in KDE, click on any of them and decide to add or to remove it.
The Font test tab can be used to preview how a font will look. Choose the font family, subtype, size and attributes and you will get a preview.
There are many entries when you click on the Tab raw X11 fonts. The KDE font manager already shows you the combinations which make sense, and lets you see which entries were useless to it. There is nothing you can do with this list except view it.