I don't think Ryley has taken a set against Tucker. As I said, they play together really happily 90% (or more) of the time. They sleep together in my room without incident, and their fighting is very limited to resource-based stuff.
Ryley is, generally speaking, a very independent and stubborn little dog. He has always seemed to overcompensate for the fact that he isn't very large by being easily perturbed/offended and by getting angry when he gets in trouble (he snaps at the pointing finger when you get after him). He grew up with cats, but after being separated from them for a time I have no doubt that his prey drive is strong enough that he'd kill a cat that got mad and defended itself from his chasing. All that being said, he is a sweet little guy.
Millie will never get involved in a fight with the other two. Never. She mostly considers herself in a completely different category than the boys.
Anywho, just a couple more thoughts on the matter. Appreciate the input
ETA - IMO, adding a puppy who was neutered as soon as he was old enough to be is a completely different experience than adding an intact male to our household. So I don't really see the connection between Nox and Tucker at all...