I'm not so sure Kanoka and Kanohi mean disk or mask, because then phrases like 'Kanoka disk' would be repetitve. But then, I could be wrong.
As for some help:
Matoran: Villager/Islander, etc.
(elemental prefix)+Metru: City District (i.e. Ko-Metru equals district of Ice)
Ussanui: Possibly "great vehicle"? I'm probably wrong on this, so don't be certain. But it could make sense. For example, 'Ussa' seems to come from Ussal crab, which could be taken to mean "vehicle crab." Again, I'm just speculating, but it does seem logical.