Actually Codex rules like Chain of Command trump Basic Rule Book Rules, same way the Inquisitor Codex Trumps Basic Rulebook rules when it comes to how a Warlord is chosen, where there is no contradiction nothing is changed so if say you have Yarrick and his rule actually read "other model" plus Creed then yup Yarrick can be your Warlord if so so choose, but add something with the Senior officer rule and Chain of Command rule would take priority over the HQ unit with the highest leadership rule.