One of the Overseers of the GenoHaradan was a Selkath so having a Selkath being a normal member of the GenoHaradan isn't too much of a stretch. The only reason why I decided to include a Selkath in the first place was because I found 1 line in the game files which sounded similar to that of a Selkath. So this guy will be able to talk briefly.
Well, most GenoHaradan assassins will wear the mask that the restored Masked Duro wears. I've recently discovered how to modify head models to allow them to wear masks but actually compelling them results in some freaky looking freaks of nature, once I get around that error, however, I will be able to make NPCs like Commoners wear Masks. The only reason the Quarren and Selkath don't wear a mask is because I think that'd be difficult to pull off.
I don't plan on doing too much with tattoos but there will definitely be diversity among the GenoHaradan.