I had a thought today. The six Rahkshi that were sent to stop the Toa Nuva from finding the seventh Toa had powers that kind of correlate to the Toa's.
Turahk = Fear, and Fire is associated with courage.
Vorahk = Hunger, and Earth is associated with strength.
Panrahk = Fragmentation, and stone is associated with stability. Stone resists better than other building materials the tendency to fall apart.
Kurahk = Anger, and ice is associated with calm, or the absence of emotion. Anger is perhaps the strongest of all emotions, and those with the elemental power of Ice tend to avoid showing any emotion.
Guurahk = Disintegration. This one is kind of reaching, I know, but disintegration is the ultimate death of something and water tends to rejuvenate things.
Lerahk = Poison. I don't know how this has to do with Air. All right, my whole theory starts to fall apart here, but I think the first four are strong enough correlations to make one think that the powers were selected to negate those of the Toa.