I installed multiple DEs myself so I basically got what you want and I can tell you:
It's not that easy. Some DEs can live with each other without problems, but some do clash in some weird ways. Also, when you install for example Manjaro XFCE it has a deep customizations for XFCE so even if you run other DE there, it may be problematic and have variety of issues, unlike dedicated Manjaro spin with that other DE.
Also, installing a DE is easy, getting rid of it cleanly is a hard work if you are not properly prepared before.
To sum it up, it's doable but messy and problematic. If you have skills you can make all DE work without bigger issues but in the end the experience is sub-optimal, so focusing on one DE that you chose with the ISO is the best and cleanest approach.
If you really want to experiement and are not afraid to mess your system you can rather safely combine:
Openbox with anything
Plasma with LXQt, Deepin, i3
i3 or sway with anything
Gnome with nothing
Xfce with nothing
Basically, Gtk based DEs are more problematic, but Plasma due its complexity also is often adventurous. Of course you can combine Gnome with Plasma, I had such configuration for a while and it can be useable, but it also can be potentially most problematic setup, depends on your skills.
Playing with DEs can be fun and a learning experience but you have to have a tenacity to work through issues and to learn a lot. If you just want to install and use, it will be rather frustrating experience as DEs are not expected to be installed together so as a rule of thumb it's discouraged.