I think it's safe to say that Nic is opposed to the Outsiders—he's evil, but he's still on the Reality Itself Home Team.
However, I do not think that Nic could take over The Gates... at least, not yet.
There simply aren't enough Nickelheads; Winter needs hundreds of thousands of Sidhe (plus various others) to protect reality. Nic would need an army.
Now, if Nic were to command the Armies of Hell? That's a different story.
See, Mother Summer said "Power has purpose," and if that's true, then it's also true that (some) Purpose has Power, and Nic has been after all the power he can get for millennia. I'd find it possible, even probable, that Nic intends to usurp Lucifer's control by manipulating events with the Holy Grail. How he accomplishes that, I dunno—maybe that's why he wanted the Lance of Longinus, to kill Satan—and then end Mab (and Winter's) thousand-year reign over the Gates by rallying Hell around him.
I'd imagine that Commander of the Gates (or Defender, or whatever you want to call it) would carry with it a hefty powerup. Nic would then get his chance to play with his toys—us, and this planet—while relatively secure in his position, as it'd be insane to try to dislodge him permanently; the Outsiders would win.