The Problem
I found the question very often but no answer in public forums: How can I use colemak as additional keyboard-layout and switch between us/colemak-kayout? Using the try-and-error-method I found out the following solution:

First Problem: editing the xorg.conf
/etc/X11/xorg.conf looks like that and it works:

Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "kbd"
        Option      "XkbLayout" "us,de,colemak"
        Option      "XkbModel" "logicdpa"
        Option      "XkbVariant" "nodeadkeys" 
EndSection

As you might have noticed I do not use the Option "XkbSymbols" "pc(105)+colemak" as it is explained in the tutorial on this page, because I could not get it working for different keyboard-languages. I tried things like Option "XkbSymbols" "pc(105),pc(105),pc(105)+colemak", but nothing did the trick for me.

Next Problem: Capslock does both: delete chars as backspace AND locks capitals...
I decided to deactivate the caps-lock-key (which I guess noone really needs) on any keyboard-layout. I found no solution to do this using xorg.conf (see above) or in the colemak-xkb-def-file (see below), so I wrote a shellscript looking like that:

#!/bin/bash
xmodmap -e "remove Lock = Caps_Lock"

and saved it as ~/.kde/Autostart/deactivatecaps.bsh (to make it executable execute a chmod 755 ~/.kde/Autostart/deactivatecaps.bsh).  This should work with KDE, but not using xfce. So I also added it to the xfce4-autostart using xfce4-autostart-editor. Reactivating caps lock inside a running X11-Session can be done with

xmodmap -e "add Lock = Caps_Lock"

as I expected.

Nice Icons and quick changing the keyboard-layout
I use XFCE4, where you have the so called xfce4-xkb-plugin. With it you have symbols for any language/keyboardlayout in your panel and you can switch the layout with a single click on it. Copying any png-file called colemak.png (where I used a typewriter as picture *lol*) to /usr/share/xfce4/xkb/flags/ will display it when you choose colemak as your keyboard-layout. The png-picture should have a size of 60x40 pixels.

...maybe anybody can be so kind to explain how this works using KDE/GNOME.

the file called "colemak" - just a hint...
colemak (definition-file for the colemak-layout) is located in /usr/share/X11/xkb/ on my system after having installed the Fedora's RPM-package, while the configure-script (which you get here) installs it in /usr/share/X11/xkb/symbols/. You might find it in /usr/local/share/X11/xkb using ubuntu/debian. You can search for it with find / -name colemak if you do not find the file.