The biggest issue you will find is getting gtk and qt apps to look, feel and integrate together. There is nothing wrong with mixing and matching, it can just get a bit complicated. Lots of people don't even use a de, and just build their own gui with a compositor or window manager, plus all the tools and apps they want to include to make it functional. I haven't used a de in over a decade on my main desktop.
Debian operating system
Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run. Debian provides more than a pure OS: it comes with over 59000 packages, precompiled software bundled up in a nice format for easy installation on your machine.
That's perfectly fine. You can mix and match. Only downside is that you get all the dependencies from different DEs as well.
That has occurred to me. My current computer has enough storage (assuming i don't lose hundreds of GBs to system log files like i did back in Mint) and i already have dependencies from several DEs from when i was trying some out and didn't know how to fully uninstall the ones i don't want.
The dependencies will be removed with the package you tried when you remove the package.