I always thought fire zones are generated strictly on a 1 per squad basis, excepting well... exceptions, like additional firezones for officers, corporals etc. in the unit.
LAMI are a good example. You get one for a normal squad. 2 for a squad with a sarge and corporal, and additional officers joining the squad give an additional firezone each.
As for the CHAS, well, he's got lots of weapons but you can't take advantage of all of them unless you alpha strike, and as I found out today he's often so far out of range of the enemy your alpha strike usually comes down to just the trip hammer and javelin.