Well, since Matoran still need to wear Kanohi to function, I would imagine that even though the power leaks out from a Kanohi made of level 6 or less Kanoka, some traces of the Kanohi's power still remains (If something conflicts this, please let me know). When Toa Power is triggered within the Matoran via Toa Stone or another source, it raises the residual power in the Kanohi up to a Great Mask level. This would explain why they bother using the proper disks, though I would imagine they would anyway, as level 6 and under Kanoka are fairly common, and the Ta-Matoran craftsmen would not want their creations to be anything less than perfect.