Seems to me that your squad should shoot in any direction without penality and don't need to have facing. If the token represents a half dozen people, they don't all need to rigidly face the same direction like spearmen in a phalanx or something. Abstract it a little, for game purposes it makes more sense. The gameplay can abstract some things as "squad tactics". Moving away from zombies while firing can be representive of a tactical frog-leap, with some individuals firing and some sprinting away and then switching roles. An individual can probably run and shoot wily-nily without having to worry about shooting their team mates.
I'd suggest a larger squad simply moves somewhat slower then an individual. It seems to me that a group of people in a combat survival situation will move more delibrately to keep the group coherant.
Perhaps larger or smaller groups could determine the attributes or abilities of the squad. A smaller squad is quieter or more efficient, a larger squad could boost morale or confidence of the individuals, better protection against zombies in melee, ect. A skill like "tactics" or "leadership" might enhance the bonuses and dampen the penalties of large groups, which might stack.
Also, I think keyboard for movement and mouse for targeting/shooting/menu would be dandy.