The questions remain of how he got the developed when the lab was not only under lock and key but also being used BY the natives in the Dark Kabuto experiments actively, why they don't have cast-off armor, and why they don't seem to go berserk.
Why can't have they been made alongside the other Zecters? Riku and the natives that mimicked Tendo's parents were working on them personally in the beginning. Either way, the order they were built compared to the others is a role, but it seems like asking about the order the Advent Decks were made. The actual hole is that we don't know why they were some kind of secret unknown even to Zect members.
Anyway, rather than Cast off, they can change forms from Kick to Punch, although that feature was never used in the tv show, likely because they decided to go with two Hoppers in the end, rather than just one like in the concept drawings for the series.
Regarding becoming berserker, they never tried to constantly protect a worm, which was what triggered the red shoes system for Kabuto and Gattack.
And is "they just lived like humans" really an adequate explanation? From the beginning of Kabuto to the end, the reality of Worms living amongst humans was the primary source of conflict...and now you've got a bunch of converted humans with the ability to mimic and that's allowed to just dangle?
Worms mimicked people and then proceeded to do killings based on personal patterns -some not even killing the actual mimicked person. The problem wasn't just their abilities. The series never really pushed forward the idea that living with them would be impossible due to their abilities- even though it could have been a significant point too. It was impossible because they had killing instincts that led them to killing other people for no reason. The abilities just made them more dangerous.
That's even more obvious with the natives. After a certain point, everyone seems to be just fine working alongside them. It only becomes a problem when their plans come to light, but before that, the idea seems to be accepted by everyone and the possibility of misusing their abilities for personal reasons is never brought up.
Now, like you mention there, the problem of seemingly "normal" people with mimicry abilities is an issue that's never addressed and could have been explored, but the show never brought it up in that sense in the first place.