What can I use instead of Sorghum flour? Is it ok to use more of one of the other flours or is there a better alternative?

You are working on ratios here -- flour to starch (and the potato flour is really starch). If you were to swap for another flour, use the brown rice as it will do the best job of preserving your ratios. I do a ton of GF baking, but don't use sorghum flour, so am not sure what taste compromise you might be making. It you can easily get your hands on white rice flour, I'd use that instead, but if you have all the others and don't want to make another trip to the grocery, go with the brown rice FLOUR rather than either of the starches.