To answer your questions...

Yes, Quash is the clan name. He goes by Vayex though.

Aria and Vayex keep in touch for a few reasons. He gets information off her as Aria sees and hears a lot of things. It's kind of Arias way of saying thanks for him helping save her life. But also, Aria knows how good a soldier Vayex is and she wants to stay on his good side because if the time comes where she needs backup, she is pretty confident Vayex will help her.

Vayex does have Paragon tendencies, especially compared to other Krogan. He has no problem killing, whether it be protection, self-defense or even some kind of vengeance but he won't kill without a valid reason. In that sense he leans towards Paragon but don't be mistaken, he is still about getting paid and has done some bad things to make sure he does including robbery, extortion and other such ventures. He was the second-in-command of the Blood Pack after all.

I definitely want to use his relationship with Aria as an ability, along with his ammo specialty, gun proficiency and krogan regen ability. What would/could be some other examples?