Regarding mosquito larvae:
I assume that you get the black ones from rain drums? So as gnats have no problems with contaminated water, you might bring a lot of microorganisms or even poisonous material in your tank with the larvae (i.e. in its stomachs).
And regarding glass worms: I just recently heard from ‘paro-colleague’ that after many of his fish died right after feeding glassworms, he did a test: he kept the larvae in his fridge for quite some time – but they didn’t hatch or even pupate at all. (From what I can see, wild-caught black and white larvae do this even in the fridge.). So he now assumes that to keep the packed glass worms alive and prevent pupation, some chemical might be added that could be toxic to our fish. (As I said: This is not my experience so far: I feed bought glass worms rather seldomly and only in small amounts and never realised any reaction with my fish. But maybe I just didn’t make the connection to this when a fish died some days after. Who knows?)