How do you determine which size of styrene to substitute for?
i was not to sure weather i read the question correctly, but this is the only answer i can come up with:
there is a thing called Scale; as far as i can recall, this model, is at a Scale of 1:96, so that would mean i would use this to determine which size of styrene to use!!!
if your trying to build something to scale then of course the 1mm thick styrene that comes with the Kit is a little to thick, but can be used, not forgetting, these Kits are meant for all modelers alike, the beginner will build it as it comes out of the Box, the experienced Modeler will cop and change things to suit the Builder, or the Original that he is trying to replicate.
i think it would be safe to say, that if your Building the Bridge (or anything else) and you want to make it lighter or rather scale thickness of the Walls, then a thinner styrene would be used, you would lose the stability a little, but it would be lighter, and to scale.
in Robins case, i guess looking at the last Photo's he is going for Scale.