The simplest way to keep things sorted is absolutely NOT by 'Users'.
You don't know how the Users feature works or what it's for, and that's why it's confusing you. If you insist on using multiple user accounts, you should read this article:
https://wiki.jriver.com/index.php/UsersThe link explains that you have to set the [User] field for every file in your library to properly implement this feature.
But I urge you not to go that route. It's folly.
The way you started, ripping everything under one user, is totally normal, and is the standard use case. Unless you have multiple humans using your PC and you really WANT to restrict those humans from access to certain tracks, the Users feature is not for you.
Chuck Berry and Beethoven will not be shuffled together unless you add all those files to the same playlist, or to playing now. Just don't do that. Don't add your entire library to Playing Now, and you won't have the issue you're imagining.
There are so many features with JRiver and so many ways to configure it, that it's easy to get started on the wrong foot.
If you're confused of how to keep files of different genres from shuffling together, then post some screenshots of your views, along with a detailed description of what steps you're actually taking to play your tracks in a shuffled manner, and we'll try to get you going in the right direction.