You can run niri from within other DEs, but I'm unsure whether it works well for your use-case. From the docs[0]:
"You can also run niri inside an existing desktop session. Then it will open as a window, where you can give it a try. Note that this windowed mode is mainly meant for development, so it is a bit buggy (in particular, there are issues with hotkeys)."
IIRC in this case the Mod key is by default Alt instead of Super.
I'm not sure, but I doubt it. You could try PaperWM [0] inside Gnome to get a feel for the scrolling WM workflow, and see if it's worth switching to niri proper for you.
"You can also run niri inside an existing desktop session. Then it will open as a window, where you can give it a try. Note that this windowed mode is mainly meant for development, so it is a bit buggy (in particular, there are issues with hotkeys)."
IIRC in this case the Mod key is by default Alt instead of Super.
[0] https://yalter.github.io/niri/Getting-Started.html