If you are going to be shaping your own scales, you can use any reasonable pin size you would like.
If you want to use 1/8" stainless pinstock, it will work fine since you will be sanding the slabs to fit. On top of that, the epoxy will fill the void around the pinstock, so you won't have much of a worry on the slippage side of the equation.
Keep in mind that G-Flex is designed to have a small amount of give and shock absorbency.
All of that being said, using the pinstock that offers the Least amount of play will likely offer some minute additional rigidity. Whether you can actually notice this small amount or not, I cannot say.