In this particular scenario, is there any reason for the Slaad to
not just stand on either side of the door and beat on the guy standing there? If the PC backs up then it just depends on whether anyone can do more than 10 points of damage per round (the slaads regenerate) when most PCs will have a hard time seeing them. Or just close the door. Presumably there's something in the room that the PCs need.
In general, if I set up this type of room I would put the McGuffin further into the room and give the slaad at least an option to hide behind total cover while still being able to attack. Or, of course, have wider hallways.