As I recall, it was implied from the very beginning, although it took a few years for the concept to be set in stone. Even in '01, all the Toa were male except for Gali. All the Turaga were male except for Nokama. All the Matoran were male, except for the Ga-Matoran. Similarly, a lot of other stuff was still vague and undefined. Ga-Matoran were established as good swimmers, but that hadn't been fleshed out as part of their elemental affiliation. I mean, at that point, even the abilities of the Toa hadn't been set in stone as create/control/absorb their element.