Drywall on conrete ceiling without studs, weight issues?
Drywall weighs 8 lbs. per inch of thickness. Plaster weighs 10 lbs. per inch of thickness. This is insignificant in the design of buildings. 

Changing a light fixture to a heavier fixture could have a bigger impact. 

I’d worry more about the kind of fastener you are going to fasten the gypsum board to the concrete slab. Typically reinforcing steel is placed with 3/4” clearance between the bottom of the rebar and bottom of the slab. 

Be careful not to screw 100 screws into the rebar and cause spalling (loss of bonding).