UPDATE TIME.
Not much progress over here.

I have been keeping Nigel and Rushmore completely separated unless Nigel has the gentle leader on and I am holding the leash. This is a big pain. But another fight between those two and Rush could be seriously hurt. I really don't know how they are ever going to resolve anything because Nigel is
definitely trying to stare Rushmore down, and Rushmore growls in response.
So, since Nigel seems to need someone to hash it out with, he goes for Zoot now. Yesterday morning I was pouring some kibble into a bin and I wasn't really paying much attention to what the dogs were up to. Well, I turned around when I heard growling, and the next thing I know Nigel has jumped on Zoot and knocked him down and would not let go of him.

Thank goodness Will was there and we finally got them apart by grabbing their hind legs and pulling them. Not too much physical damage to the dogs, but quite a bit of mental damage to me and Will.
Then again this morning, I was weighing Nigel by picking him up and standing on the scale (not easy) and as I was setting him down he started to lunge for and attack Zoot.
Well, I think Zoot finally got the clue that he needs to get out and stay out of Nigel's way. Since this morning Zoot will not look at Nigel and won't come near him. I am hoping that is all it will take for Nigel to leave Zoot alone.
As for Rush, I just don't know. We have been keeping him protected from Nigel, and maybe we can get used to this, but I would sure rather everyone could be in the same room!
Samson has not had contact with my dogs since my original post. He has been completely separated, and I think he will be going to his new foster home tomorrow or the next day. Nigel is doing EXCELLENT on our walks together, and he really seems to enjoy the one-on-one time with me. He has been more obedient than ever and is very happy to sit and wait for his food and to go through doorways when it is just me and him. But put him with all the others and he is rushing to be the first through the door. Oh and it turns out Mokey was in heat (which couldn't have been helping anything!) so she went to NC to be spayed by Will's dad and she'll be there all week.
The conclusion seems to be that since Nigel is turning one he is really just wanting to be the top dog. I have read a lot of articles on "sibling rivalry" in dogs and it sounds like ours is a very classic case. Nothing out of the ordinary at all. Boy was I freaking out thinking he was a psycho. Not that our situation is any easier, but I guess it is good to know that this is "normal".

However Will and I feel completely worn out!!!