Siskos prejudice?

[Poltargyst]

Having reviewed the dialog, I don't think Sisko is being prejudiced when he brings up that Nog is a Ferengi. He is pointing out that the Ferengi live under a certain philosophy that is incompatible with the ideals of Starfleet and is asking why someone would want to join an organization with different ideals than those of his native culture. "Where's the profit in it?" This allows Nog to then explain himself.

 

[Shamrock Holmes]

We know from The Drumhead that Starfleet servicemen enjoy rights similar to what American citizens enjoy and, in an investigation, could refuse to answer.

It's very unlikely that Nog would be able to invoke the "Seventh Guarantee" as it is specifically requires that the evidence that they are being asked for be self-incriminating* and he's not being accused of any kind of external offence, but potentially committing the offence of "refusing to co-operate with an investigation"*.