My opinion: They’d probably use their birth gender. Like a wizard born a male, chose to come out as gender-fluid, for example, would probably be sent to the male dormitories. Unless they personally ask to be in the opposing gender’s??

During the HP timeline the above but later I'd say they would get there own dorms.

chose to become gender-fluid

But... you don't choose your gender. You're born with what gender you want to be (not physically, smh, I'm trans too), it can just take time to figure out for some people. Nobody chooses to be LGBTQ+, they're born with their preferences (for gender and/or partners). Everyone is. If I could just choose to be straight and cisgender, I probably would, to avoid the oppression. But I'm just not.

Better wording would be choose to come out as Genderfluid

@InsertWittyNameRightHere My sincerest apologies if that was offensive. I was born a girl, and believe that I completely am, so I’m not really use to the LGBTQ+ things, though I support it. I’m just trying to say that like... you are still legally born a gender, you just may not feel like it? Do you get what I mean? Like, on a birth certificate; ‘male’ or ‘female’. It made a lot more sense in my head... :(