@Fizz in the end you just install it and at the login prompt change you DE with the drop down menu.
"sudo apt install kubuntu-desktop", logout, change gnome to plasma in the drop down and login. That's how you switch from gnome to kde on ununtu. Be aware that this might install some KDE services that might run in the background though.
@Fizz Thankfully it is. Biggest problem might be that the installation of kubuntu-desktop changes the login manager from gdm3 to kdm or that some kde services are running even if you are logging in on gnome. Other then that, you can just install about any DE you like on any distribution, if it is packaged in the corresponding repository