On Scorpius being an only child: I actually doubt it. I mean, if you take a look at the Black family tree, there is only one only child- Draco Malfoy. I think that purebloods like to have a few children, like three or four, but look down on the Weasleys who have more children than they can afford. I think that Draco was an only child either because his parents were unable to have any more, or because they just didn't want others. I sort of think the first one. Lucius was always setting high standards for his son (remember in CoS in the shop in Knockturn alley?) and Narcissa was babying him. I think that this points to the fact that they were unable to have more children. Therefor, I would definitely think that Scorpius had siblings.
On Slytherin House: Rhi for HP is right, that prejudice does come from canon, and the fact that Harry himself is quite prejudiced. I think that people sometimes forget to think about the qualities that put people in Slytherin. The qaulities for each House can be put to good or bad use.