Given the fact that there was enough Toa for Lhikan to lead an army of 300 of them, I feel it stands to reason that Matoran destined to become Toa are more common than we were led to believe. (I’m not counting Toa that were never Matoran in the first place such as Helryx, Orde, the Toa Mata/Nuva, and (presumeably) the first Toa of every other element.)
Matoran have no problems drinking through their mouths. It’s been established that Matoran are 85% mechanical. Machines with moving parts can potentially overheat from too much use. A good way to prevent that is a coolant (a drink). Maybe their mouths are the only really effective way to get the coolant into their bodies. They mainly drink water. So, it would hydrate their organic tissue while simultaneously cooling and lubricating their mechanical parts. The water used to cool the mechanical parts would likely evaporate from the heat and be expelled through their mouths as they breathe.
Edited for Double Post - BioKnight