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.
Nil punning in Clojure gives you that kind of experience, for example. Things that would break in other languages, just "work as you'd expect them" in Clojure (except when you drop down to host primitives, and then nils don't behave nicely anymore). In general, it makes for a really pleasant dev experience, I find.
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