Hanging chandelier

[JohnW2]

I think the thinking is 150 to go into the size of that hanger, doesn't need to be more than 50x50,

It wouldn't even fit into a joint hanger, since they are designed for 'joist width' joists - so 150mm x 50mm would fit an appropriate hanger, but not 150mm x 150mm.

Anyway, as I and others have said, the OP's proposals represent very serious overkill. 9kg is pretty trivial - almost any screws into any sized bit of wood would be adequate to support it.

Kind Regards, John