I’m not sure if a zipper is required for FS, but it does make it quicker to put on and take off. The FS aspect is more for fire retardant materials and anti-static properties.
It shouldn’t be roomy though. It shouldn’t be skin-tight, but it shouldn’t be like a freaking sail which is what some folks’ CADPAT uniforms look like because they’re sized in 4” increments.
CCUE and the upcoming CADPAT MT are in 2” increments so people like me who are in between the current sizing increments don’t have to pick between “too tight” and “too loose”.