Hi
I think the easiest way is to just create or use an existing field in your database. Simple example use the field Grouping which already exists (if you are not using it already). Go to a File "View" in Standard View and select all the files for Classic ->Open the tag Window (CTRLALT+enter is the shortcut) type in Classic in the Grouping Field. If you don't see the grouping field the first time use the little icon in the top left corner of the tag window). Repeat for Popular and Opera.
I'd recommend for the first custom views you create, you do it in standard View, not Gizmo directly. Choose Add a View by right clicking in the audio part of the tree and choose type of view you want to represent which would be categories I would think -- here you'd want the Genre template.
Then in the customize view box which is now open click add towards the left and choose the Grouping field. Move it up to the top of the list. Then remove, change or add to the box on the bottom left. This will allow you to drill down -- if you want to skip genres and go directly to album artist remove Genre, etc. I'd recommend unchecking the box to support tree browsing in the middle pane as you won't be using this -- your choice. Check out your view, if you like it go on.
Now go back to your customize Gizmo menu and click add=>from standard view and choose the name of the view you just created. (put this under Audio though). Of course you can get more elaborate than this eg. Category (using the grouping field or one of your choosing) = Classical Genre=Lyric Style=Opera Period=Baroque etc etc etc.
I'd recommend to insure that write tags to file is checked under the grouping field, btw. (ses options Library)
Baby steps -- all you have to do is remember to tag the grouping field when you add new media and you are set to go.
Remember golden rule number one --> backup your library often, backup and even archive your music often. Then play around and have some fun organizing your collection.
Also, I see so many people relying on choosing physical files and not using the tags and the power of JRiver highly customizable data base. Use the metadata for your views and you won't have the problems you are describing any more.
Hope this helps