If a light fitting needs to be fixed between two joists..a block of softwood should be fixed between them to take the weight of the fitting.
Having worked out the position for the rose, remove the relevant floorboards in the room above the ceiling and cut the block to fit between the joists. Nail two pieces of softwood into the block, flush with its ends. Mark the position of the rose and drill a hole sufficiently large to take the wiring. Then secure the block by screwing through the side pieces into the joists. Assemble the light fitting and replace the floorboards.