I have 4 dogs so I know what you mean. My top dog Sam and she often puts the other dogs in there place but nothing like what you have described. I have never been quite sure what to do but have now decided that as I am the top dog and I can tell all the other dogs what to do and if I don't react then the dogs will assume their behaviour is okay. I suggest that next time Sniffy shows any aggression to Pepe Sniffy should be told to stop and taken into a quiet room and left there for 5-15 minutes. I think what you have described is very scary. All the dogs ganging up on one other dog is not a good thing. Show Sniffy you are the boss and that you will not let Sniffy go after Pepe but still treat Sniffy as the higher order dog if that is what Sniffy is.