Donot hesitate to lay the slab of first floor during monsoon.There is no any chance of settelment at first floor during monsoon as in the case of ground floor.
Extra Precaution in monsoon to arrange 2nos tarpoline, if sudden rain then cover the cement and laid RCC slab. If there is rain than after the rain finish the surface of slab with neat cement with iron float. The other precaution is to arrange good quality material, sand should be coarse sand with silt note more than 4%. Use fresh cement.