Meshing the slab is important as you said. However please note that if you are modeling the floor using plates having appropriate thickness, you are already accounting for the stiffness due to the flat slab and so you do not need to define master slave on top of it. At column locations where there are caps, you can add a few dummy beams ( use high E and zero density for these ) connecting the column point to the adjacent nodes of the plate mesh. That would simulate a stiff cap and help distribute the stresses.
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