@King:
Kiku considers herself female. She stated as much.
Izo is another case entirely. He may just be a crossdresser out of respect for his father's legacy instead of something more self-styled. We don't know much about him.
Kiku has only really stated to be a 'woman at heart'. That's no real confirmation or denial and could be interpreted either way. Either 'Yes, I am a man, but I am a woman at heart' or 'I may have a penis, but I'm a woman at heart' For all we know, he's not denying his biological gender/sex. As far as I'm aware, Kiku also uses the male version of 'I' (sessha, which is less common for females to use than 'ore', like Big Mom does). I'd say, considering Izo was clearly stated to be male as well, I wouldn't get my hopes up about Kiku being trans.
EDIT:
@Captain:
As has been already said, Izo and Kiku are unique cases. There's a difference between a trans woman and a man choosing to wear traditionally feminine clothes (even if most of the difference comes down to the individual's self-identification). Kiku is one, Izo is the other, and I think Oda's made that pretty clear.
I said in my larger, earlier post that I don't like that Oda muddied the waters by setting it up that way, even if I'm sure he thought he was clarifying the situation by doing it. Time will tell if the odabox is a sign that Yamato doesn't care how he's referred to, or if it was just to make absolutely clear why there was such a difference between his appearance and identification, even if he doesn't see himself that way anymore. In a way, it feels like pulling a birth certificate or a government-issued ID on a trans person, even if Yamato isn't exactly that. We'll also have to see what Oda himself does going forward, what third-person pronouns he chooses for Yamato if he ever talks about him in an SBS.
My values emphasise giving people the benefit of the doubt when they say they want to be referred to a certain way, not trying to dig up some "gotcha" proof that they weren't born as what they want to be now. I'm pretty firmly set in this being the right way to treat people, so yeah, I'm always going to be a little suss on people who wouldn't do the same. There is something to be said about the distinction between fictional characters and living people, and that idea that fictional characters aren't due the same respect as real people. I myself have argued often enough that adapting a character's name in a translation can be the right thing to do to make a story or scene work, even though it would be enormously disrespectful to do the same thing to real person. Buuuut at the same time, reactions to a character can often be telling of how a person really feels about people like that character and how they'd treat those people in real life.
(And to be clear, when I say I'm being slightly judgey over it, it's more of a 'narrowing my eyes at them from across the room' kinda thing than it is picking a fight with every single person using the wrong pronouns. I don't have the time or energy for that many internet arguments. But I promise that if Oda clarifies Yamato's identity in such a way that respectfully opens him up to different pronouns, I will completely rescind all my privately-held scorn about everything said in this week's discussion.)
Don't get me wrong, I completely see your point, but Yamato's female mask, generally feminine face and just the way she's been drawn make it pretty obvious to me. If Oda had wanted her to be seen as a trans man, I am pretty sure he would've drawn her with less…feminine features overall. Adding the kabuki themes with both Yamato and Kiku, it seems like they're references to Onna kabuki and Wakashū kabuki, respectively. So to me, it just seems like interpreting them as trans seems like a very western thing to do. I'd completely agree with you if this was a western medium.