"I smell macguffin in the wind..."
Giant werewolves would be cool.
Ability to tear through magic/spirits is also cool.
How impulsive would they be compared to regular werewolves? I imagine them as wiser and more deliberate than the typical werewolf (in examples of Fenrir talking, his demeanour is oddly calm). They could still be ferocious in this case, just with the self-discipline and intelligence to match.
Didn't know that Fenrir is actually a frost giant. That would be interesting to allude to in the Fenrirkin's abilities.
Lets see, weaknesses...
The tale of Fenrir suggests that they should be vulnerable to.... an enchanted boot to the face.....ummmm....
Since they seemingly excel against magic and the otherworldly, what if they were more susceptible to mundane weaponry?
Lastly, Fenrir was sealed with a collar, so the Fenrirkin could also have collars that inhibit them in some way (and presumably cannot be removed--until that condition was met where you mentioned them getting huge). Gleipnir was made to be "thin as a silken ribbon", since Fenrir broke through the strongest metal chains. It was made of "six impossible things" which to me is more evocative of its magical nature than any specific thing. Could be interesting.
EDIT: also, I don't know the tale, but maybe Fenrir sounded so polite because his rage was being restrained by the collar? Maybe that is the case with the Fenrirkin? The collar actually inhibits their aggression, possibly even making them weaker than regular werewolves?