you cannot install multiple DEs without breaking stuff.
You definitely can. There is no problem on having, say, Plasma, GNOME. XFCE, and so on on the same machine, even on the same partition and switching from one to another from the login menu.
[it is not a good idea to have] different versions of the same DE
This, however, is absolutely correct. The overlapping different versions of libraries and other common software would make things very messy indeed.