You already guessed, mix of DEs will dirty your system (especially with Qt KDE mixed with GTK Cinnamon) and can be prone to problems/conflicts in the long run.
You want Cinnamon, install a fresh clean Cinnamon…
Just, why?
You have also the possibility to customize your installation with Architect. Or even, if you feel brave, through manual install with terminal.
Best approach is to not run multiple desktops in the same install. Regardless of claims otherwise if you want to run multiple desktop environments then dual boot.
Do note that a major difference between installing Cinnamon from another DE and straight installing the Cinnamon spin, is that the spin comes with a default selection of applications you won’t get by simply installing the cinnamon packages; you will need to also select those applications for installation.