I always like to headcanon that the differences in head shape are because some of them wear Doom Viper skulls as helmets. Reasoning:
- We know that in-canon (thanks to Tobduk’s staff) that Doom Vipers have skulls.
- We don’t know the island of origin for Doom Vipers, and it seems like a good pick for such a species
- TSO has that same piece integrated in his armour. Of the four, Conjurer is the only one who doesn’t wear any, and he’s also the most oddball of the bunch in-story, so kind of makes sense.
I don’t think they’re too different to class as a single species. They’re no more diverse than, say, Toa are.
Metallic Hot-Pink Pouks incoming. 
I’d actually be on board with this.
I’m kind of against this. By the metric of “did they appear in the story” - many didn’t. We know they must of had other forms, but the whole point of giving visual depictions to characters is to better visualise them in-context. WHen all we have from them is word-of-mouth tales for other characters, it doesn’t make sense that we should get those designs (at least, the way I see it).
There’s these things (basically Thornax launchers, but still) in the Core War panel for Mata-Nui sage that I always imagined being Force Blasters.