I updated to JAVA 11, to find out later it will not run all my Servers. since a plugin I want requires this upgrade, is there a way to run JAVA 8 and JAVA 11?
If you have both installed, you can use
sudo update-alternatives --config java
to switch between versions as per your needs.
SO I can’t have one server running Java 8 and rest Java 11?
From the OS perspective it is possible to run several minecraft servers with different java versions. You “just” have to start the server with the java call referencing the binary for the java version you want to use. It’s not quite straight forward, but it is possible: centos - use different java version to run two programs - Unix & Linux Stack Exchange
MineOS however cannot do this. MIneOS is a WebUI that is layered above the OS, so it uses the default (main) installation of java it finds. MineOS needs to be told to use another version of java, and there is no way to tell MineOS this in the current version.
It might be a feature request for MineOS to have it detect installed java versions on the host, then list them in a dropdown menu in the minecraft server settings to select what version to use. It ads another level of complexity for the users of MineOS though, as they now need to be aware of what java version each minecraft server version needs.
So the answer to your question is that it is possible, but needs to be done manually, and the minecraft server will most likely not be seen as active by MineOS.
My issues is my forge server will not start due to it requires JAVA 8 and I updated to Java 11 to all a plugin to work right, without knowing it would prevent my from running my forge server. I suggest if no version is chosen then by default use latest version installed