The confirmation we have is that there exists a faction called the Hippos, and that Yuna's enemies were from that faction, but that doesn't mean it is THE scum. Agreed, it does very much look like it, and I think I'll work on that hypothesis from now on, but let's not forget that both Stephen and Yuna have win conditions that lie outside the normal mafia set up. It's still possible there are other groups or factions, or that certain players/factions can win without the Hippos being eliminated (the standard definition of Scum). You yourself say you can win when a certain amount of nights go by without an NK. At that point, we may assume that it is not necessary that the Hippos have been eliminated. In particular: I'm not sure if non-Watch townies would win if you or the Watch would. I dont think I'm willing to risk that.
But even if we now would have had confirmation the Hippos are Scum, and I agree that it's a good place to look, how to find one? Races don't really matter as Taiga pointed out, as non-humans are not always foreigners, and humans are not always Ankh-Morporkians. Plus: we don't have that kind of information, except for one. The one person we do know is Ankh-Morporkian already is ranagrande, but I have no idea if he can conceivably be considered a xenophobe. So far, from the descriptions of Stephen and Yuna, the roles and characterization seem to fit the character nicely, but in order to "make" scum, Isaac may have found it necessary to "tweak" the characters a little.
I understand that I look like a good target to you, but you'll find my voting record to not be strictly anti-Yuna. And what I said at the start of today, was the following:
If that doesn't signify a group (and a dangerous one at that), which I have been thinking about since the beginning, I don't know what else it is.
That's not going after her perse, but indicating that I would eye her extra carefully. And I did, if only she would have reacted to questions of either roleclaim, or a little more information. In addition, you DO know I didn't do the NK on Stephen. After all, I was detained by the Watch (or, to be precise, by Praz, which still is a strange story) during that very night.