Actually due to the nature of a spoke being a tension member - it strictly transmits tension to the rim - so gauge and shape are meaningless (outside of having to know them so that you can interpret the readings on your tensiometer). The optimal tension for a build is determined strictly and solely by the rim itself.

Each rim is different, but in general most modern rims can withstand upwards of 120kgf or even higher. I tend to shoot for more in the 100-120kgf range....again - dependent upon the rim itself. In general the tension does nothing to stiffness or any other property of the wheel, but the appropriate tension will support the rim and allow it to endure longer.