First Founding wasn't a point you raised. And where is the rule for that, that only First Founders could have a codex? Especially when the Templars already had one?
And for their codex, as I said, they were almost as codex unadherent as the Space Wolves, and pay just as much attention to it. Quite unlike the Codex-adherent Dark and Blood Angels.
And this is GW's words, not mine.