Hi Dave, and welcome.
I wouldn't bother trying to manage multiple libraries. Instead, just change the views to either show only the file types you want, or better to present the different types in a way that you can select the one you want (depending upon how you want to browse and work with your music).
If you truly don't want to import your lossy files, then configure Auto-Import (Tools > Import > Configure Auto-Import) and remove any paths that point to lossy stuff (presuming your stuff is organized that way). Or, select a path there, and Edit, and disable certain file types you don't want imported. Once Auto-Import is configured, you can go to the Audio > Files view, select from the top-right pane all the lossy file types, and then in the file list below, select all the files and hit Delete (leave selected the first option in the dialog that will appear).
If you do want your lossy items in the Library, then its easy enough to modify or construct a view to either allow selection of types, or to show only certain types. Maybe you can comment on the method you'd prefer.
