There are ways around the messiness of having multiple DEs involved if you have a seperate user account made for each DE.
I've experimented with all of the DEs here. There were no lasting issues from having Mate/Gnome/Budgie installed EXCEPT I was never sure how to switch back from GDm to LightDM.
I installed KDE on top of all of that (I was planning a reinstall anyway and wanted to try KDE out) and I did run into problems which really borked everything up. However, I was able to circumvent these issues by making a user account for each DE to login individually and deleting my original account. New Users are great in Solus; it almost like a completely fresh install!
There were still a few annoying issues though in KDE (the wallet kept popping up) and I went ahead and reinstalled completely and now have only KDE on that laptop. My original install of Budgie has been running without issues for over a year now on my main rig.
There was no real reason for me to do anything of this except to see if I could. I wanted to understand how the libraries worked together and how they clashed. I have a much better understanding of DEs because of all of this and I FINALLY understand why Flatpak's are preferred over software from the Repo's if it is QT and you are running a GTK-based DE, or vice versa.
Pretty sure if I had just made the accounts from the beginning that I wouldn't have had those issues. But there will always be unintended consequences of doing stuff like this to your system and if you want to prevent problems you should -never- install KDE alongside any GTK-based DE in Solus.
