Normally, no sealant of any kind is needed for the oil pan drain plug. A good plug with a new copper O-ring should seal perfectly. I'd wonder if your mechanic supplied the correct parts...? A torque wrench normally isn't needed, if you know by "feel" how tight to make the plug. If you're not sure, a torque wrench may not be a bad idea. Woudn't want to strip the plug again...!
