yes, LR 5 catalogs are compatible with LR 6. In LR 6, open the LR 5 catalog, use File->Open Catalog
Note: the other way around won't work, LR 6 catalogs cannot be opened by LR 5 no matter how hard you try.
Depends on your definition of "compatible".
LR6 knows how to convert an LR5 catalog to the new LR6 format.
LR5 cannot use an LR6 catalog.
LR6 cannot directly use an LR5 catalog.