The basic design "rules" for the members of Noosehead are their height, their matching noose ties and jumpsuits, and their sort of spherical Felix/Mickey-esque heads... white skin, black hair, black (or at least 'empty') eyeballs, no noses, no eyebrows, and no ears. (The triangle player, of course, is totally exempt from all of this, since the whole joke with him is how he looks nothing like the rest of the band.)
That said, when I launched volume 4 I made the decision to redesign Nicole to make her to look more human than the other Noosehead members... mainly because she was going to be romantically involved with Sam and I didn't want them to look too jarring together. So that's why she has a more normal-styled head, with just a few traces of the Noosehead design elements. (As opposed to how she looked in that old drawing Kelso posted.)
As time went on I also tweaked both Nicole and Sid's designs a bit to give them a broader (and subtler) range of facial expressions. Nic and Sid's rolls in the story are a lot heftier than the other, more gag-driven band members, and they needed designs complex enough to handle that.
You are hearing me talk.