It is a mistake to think that Marinelli's system dictates only "Ultraquick" DTs. That's actually not the case at all. His system relies on players who are disruptive. Now, this can come in a few different forms. They can be exceptionally quick or they can be massively strong or they can be a combination of both. Nix is the latter. I would love to have Nix and I believe he is the best DT option in this draft but again, that is only my opinion.
I would be very happy to have in at 16. JMO
Let me put it this way: I seriously doubt we will ever see Marinelli with a starting 3 Tech who weighs 330 pounds. While I do agree that Nix is quick for his size, he isn't nearly quick enough to play Marinelli's 3 Tech spot. There are many guys in this draft alone that could be had outside of the first round who have better quickness and would fit that position better.
Let me put it another way: I seriously doubt Dallas drafts Nix in May, because if he was going to play on the line for Marinelli he would almost surely be a 1 Tech and Dallas has already come out and said they think they can find that position after the first round.
Let me put it yet another way: Dallas has said this off season that they want speed and quickness over size, and that it is those quick-twitch traits that they want to try to bring on board. Imposing size has never been a hallmark of a Marinelli or Kiffin defensive line.
So, when the team says they want quickness and speed for their defense (even at the 1 Tech), and when the team says they don't want to draft a 1 Tech in the first (even if they did want size first), why would you argue that they will take a guy in round 1 who is not quick or fast as compared to guys like Jernigan, Donald, Reid, Easley, etc.? Some of whom can be had in the second round or later.
Remember, the OP asked if anyone thought the front office was interested in Nix, and that is what I was referring to when I said I didn't think he fit the system or was what Marinelli wanted... which you said was a "mistake" for me to think.