I know it used to be possible to install several different flavours of Ubuntu into the same Ubuntu base. I could log out of flavour 1 and log back into Flavour 2, and so on I have forgotten how to do this :-( Please advise.
Yes you can install multiple desktops on a base system.
The system I'm replying to you on is originally a artful (17.10) Ubuntu desktop install, on which I added
ubuntu-mate-desktop. It's been upgraded every six months (I don't remain on LTS or long-term-support releases).
I have since removed
ubuntu-mate-desktop (because of disk space issues; I used it least).
- On adding the second desktop I would not expect any issues.
- On adding the third desktop there is a minor chance of issues.
- On adding the fourth desktop there is a substantial risk of issues.
- On adding the fifth desktop, issues are to be expected.
I installed my 4 (GNOME, XFCE, LXDE [now LXQt] & MATE) many times in different orders before I calculated the best order without issues, then formatted the system and installed them for real (and it's still running now, where I'm replying from).
I also wanted KDE (or
kubuntu-desktop) but decided that was too much, the problems got annoying.
It's not just the desktops, but also the order they are added.
The above is from my experience, as a multiple-desktop user for over a decade on Ubuntu (longer on Debian).