That one dude has a wicked tiny head compared to his buddies. I'm not a genius when it comes to designing characters either- but I think the two main points for coming up with effective character designs are 1. make 'em look unique and 2. design the character's outside to reflect the inside. This will create iconic designs that the viewer can instantly associate the specific character with whenever they appear.
So yeah, uniqueness is pretty important so you can quickly tell them all apart and have that immediate association. Simple palette and clothing changes over the same basic mold will still give you a bland, Scott-Pilgrimesque look. The noses and head shapes are all different so good job there. I suggest messing around with different heights, weights, and builds to add some real variety.