Well Yamato hasn't joined if this is refering to the SBS, whcih let me repost for the thread:
D: Heso! A question! Luffy gives nicknames to all sorts of people, however, why doesn't he do so with his crewmates? P.N. Takataka
O: Ah, I see! Well...Luffy's a type of person who, basically, doesn't really care about remembering someone's name, so he'll give people a nickname based on his impression of them whether they like it or not, but with that said, when someone becomes his crewmate, he finally remembers their name. He knows their names, so he calls them by their names. That's really all there is to it.
Ultimately Yamato hasn't joined yet so I don't thinks this really changes much on weither Yamato will join or not, hell Luffy didn't call Chopper by his real name all throughout Drum Island only referring to him as Reindeer.