No, The SI being pureblood makes perfect sense. It actually fits the SI story even moreso then other races imo...His/her Ancestors being disgraced Sith for countless generations till the SI comes along and returns the family name to glory.
But why would he be a slave? And why would Fenn or whatever his name was be held higher because
he was a pureblood. Purebloods don't become slaves, no matter how disgraced their ancestors were. Their is no evidence to support this. This is why BioWare originally didn't include purebloods as a species for SI.