In ship design, where are the life support and grav plates accounted for?

It's assumed that these are two ship systems are, or need to be, distributed (throughout the hull).

Are they centralized?

Gravitational plating would indicate that artificial gravity is not, and for most primary hulls, it's all or nothing.

Could it have a rating, instead of the assumption you could ping pong it by, presumably, six gravities, of maybe just two gravities, and thereby cheaper, who knows?

Life support is presumed distributed by default, since you don't need breathing apparatus in the depths of the cargo hold, though that could be accomplished by an efficient ventilation system, and an overclocked life support generator.
High Guard pg 51 Multi-Environmental space seem to fit what you were looking for. If you’re looking at a cargo box with nothing else but you want atmosphere that would be the best way to figure it. It even makes sense because you’d want the ability to carry the largest variety of cargos