It is really personal preference. You can make flexible parts that look solid with the right materials and techniques. omarbou1 is right about the abs/midsection. I made the model's midsection split into overlapping sections to allow more flexibility. Some people prefer even more free movement which is why some people do their suits completely in foam.
Yes, if it fits you well that way then it should be the same scale or slightly larger scale than the upper forearm.
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I am hoping to have the new Mk VII helmet done for you all today. Keep posted. Yes, I am sharing this V2 file with everyone right now. I am only sharing this V2 file. I will not be doing this with any other V2 files. I am only doing this because there was never a V1 helmet.