It's the distinction between a "window manager" and a "desktop environment" KDE/Gnome/XFCE are DEs that include window managers (KWin/Mutter/xfwm4) along with a suite of other utilities that make up the complete environment.
Conversely, Sway, Niri, Hyprland, i3 are bare window managers. They do not include the suite of tools and it is left up to the user to build their environment as they wish. Fortunately thanks to some defined (FreeDesktop.org & Wayland are big) and defacto standards there is a reasonable degree of interoperability for tools. For myself I pull a decent chunk of the XFCE suite into my Sway config to make my very own, special little environment. A environment that apparently no one else can even begin to figure out how to use but at least nobody asks to borrow my laptop twice.
Mixing can work pretty well. I'm using Plasma with i3 as a WM, and it hits the perfect spot for me. Not sure if the same thing can be done on Wayland, though?
I would expect it to mostly work. Standardizing the interface between the window manager and clients. So as long as Plasma isn't depending on any special behavior (by intention or assumption) of KWin it will work just fine.
Conversely, Sway, Niri, Hyprland, i3 are bare window managers. They do not include the suite of tools and it is left up to the user to build their environment as they wish. Fortunately thanks to some defined (FreeDesktop.org & Wayland are big) and defacto standards there is a reasonable degree of interoperability for tools. For myself I pull a decent chunk of the XFCE suite into my Sway config to make my very own, special little environment. A environment that apparently no one else can even begin to figure out how to use but at least nobody asks to borrow my laptop twice.