Hm... What's my tally on post-install added DEs, again?
I can tell you, WITH YEARS OF EXPERIENCE, that it's perfectly safe to install other DEs after installing one edition. (As 'perfectly safe' as the distribution is in general, at any rate.) The thing you need to watch out for is installing all the components you may need, or you won't be anywhere near the Mint Cinnamon experience. That's all, really. There are default settings and desktop/panel layouts that you shouldn't miss out on (or you'll have to set up the new DE on your own; believe me when I say that installing KDE manually (i.e. not selecting kubuntu-desktop first thing) over an Ubuntu GNOME installation isn't for the faint of heart), but there are no laws or defined best practices.
A DE is a DE - not strictly speaking a system component. If you remove one DE, software designed to work with that specific desktop will, of course, not work - but any sane package manager will set things up so that packages depending on one DE will be removed along with the DE, if THAT is removed. (You'll only observe this with GNOME and KDE, though - most software in Mint uses GTK+ 2 or 3.)
Just... One big JUST IN CASE: IF problems arise and you can't log in (double check your session in the display manager!), do not fear the terminal (Ctrl + F-keys). It'll have full internet access, so you can install/reinstall/update packages from there.
Tinker to your heart's content, but try not to get carried away.