I have found the stock coupler screws on my Spectrum 0-6-0T to be weak and generally unsatisfactory. #4 brass machine screws work much better, plus there is a choice of drives.
Many of us use 2-56 screws for couplers. If a screw happens to be Metric, I re-tap for 2-56 machine screws. I keep some nylon type screws on hand for mounting the motor in frame halves for isolation. I put a layer of Kapton tape between the motor and frame.
I keep 00-90, 0-80, 2-56, 4-40 on hand. Very seldom ever need a 4-40 size in HO.
I seldom use a sheet metal type screw as it sometimes splits the wood or plastic if the hole is a little too small in diameter.
Rich