If you want plaster ceiling to hide the beams, then I dont think you should since the ceiling is already low.

If you want the plaster ceiling coz you want to have downlight, then you can have plaster ceiling at have lesser depth - just enough for 4 inch depth downlights..meaning plaster ceiling don't cover the beams so you will still have sunken parts with plaster ceiling from section to section...and just add cornices around the beam area..

For fan, you can leave a whole where you want the fan to be...maybe round hole instead of square hole so that your fan sits on your original ceiling instead of the plaster ceiling...