I'm taking the 40k plunge and have narrowed down my choice of armies to either Grey Knights or Black Templars. All is good with the GKs AFAIK, they're a newer army with no compatibility issues for the 5th edition rules. The Black Templars have an older codex that pre-dates 5th edition. Are the BTs still compatible with 5th edition as their own force without being generic Space Marines? Is the 2005 BT codex still a valid document and are all older codices that have not had a 5th edition revision still usable?