Your experience with Nox should have prepared you for what might happen this time.
Although this sounds harsh to hear, I agree with this 100% and I would encourage you not to get another dog if you find that this situation also doesn't work out. Nox became aggressive with the rescue and you did not solve Nox's problem so it's reasonable to assume that there was a risk in this identical behavior happening again - and it has, albeit occasionally.
If I were in this situation, Rylie would be separated from the new dog 100% of the time when I was away from the house or couldn't intervene in case a deadly fight or serious wound fight did break out. The signs are pointing to this possibility - you are in the risk zone.
as Goldfields correctly pointed out - our goal is to respond to the important title of "Trying to keep my neutered males from killing each other".
I DO 100% think that training can help this situation but I'm afraid i don't have the specific training advice you need in this case - curious of course, to read what other members may suggest.