Re: Uninstalling previous MPLab X versions
Depending on your needs, even only one could be all you need.
There is no need of having installed versions you won't use. I usually keep 2 latest versions of MPLAB X (after I download a new released version) and once I'm comfortable with the new version, I then uninstall the old one.
You can always install an older version when needed.
For compilers you can also keep the old ones depending on the kind of projects you are working on, compatibility with your old projects, plib support, etc.