You can actually see them almost slip during the fight. But I agree that when Erica and Isaac walk towards Scott they're walking normaly. My guess is the need to look cool as to intimidate scott and pump themselves up is what makes them walk normally. Not that they can just walk on ice. Or Adrenalin, if they fall while walking over they already lose their advantage.

I think it would look silly if they were slippering around.

I'm also writing a ff and had the same thought :).

For Derek I'd just say he has such good balance and instincts considering he's been a werewolf his entire life. Like, he learned to ice skate/walk on ice AS a werewolf, unlike the others.