But still, I will always see Zoro as First Mate. He's the one who doesn't get emotional, for example the fight between Luffy and Usopp. Also the thing with Kuma, don't know if it's a good example. AND he told the team not to rely on Luffy's Powers only when they fought Oz.
It's true, Zoro was never mentioned as first mate except for here;
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Could be translation or whatever but Zoro will alway stay First Mate for me since he's also the second strongest/on par with Sanji.