Originally Posted by Brother Lucian
I think you hit it in the nail that the Salamanders is meant to police and mind the astartes themselves, to keep them in line when the more volatile legions threaten to go out of control.
I think you're on the right track, but it has been pretty much confirmed that the SWs were there to police the other legions. Just look at who tackled Angron when he was out of control and slaughtering millions. Despite the SWs apparent lack of inhibitions in doing similar if the cause deserved it, it was Russ, not Vulkan who stepped between the WEs and the innocents.
You made a good point about Curze taking the burden on his own legion so that the Sallies could be the good guys. Maybe it's in a more figurative than literal sense that they are meant to be the most human. Post-humans that could be held up to say, hey, we're not that different from you. Except for the black skin and glowing eyes.
That doesn't really tackle the bit about them that says "they varied considerably in gene-forged ability to their peers." We know they seem to be slower, but stronger than other legions, all appear to have a streak of Vulkan's artistic and smithing talents, and that they have a penchant for burning themslves with hot brands, despite being highly resistant to heat and fire.
I can see how the SWs sharper senses and faster reflexes (this is also stated as fact in Inferno) might give them the edge over their brothers if required, but the Sallies baffle me.