Super interesting question. Something like throwing-up just isn't in the cards at those scales due to the lack of inertia and overwhelming electrostatic, Van-der-waals and other effects. (Take a set of tweezers and try to do some kind of art at those scales and you will know what I am talking about)

Even the air feels like going through molasse to an insect according to Kurzgesagt, which do research their stuff.

Yet it seems getting food out of their stomach is part of their nature:

>Ants have two stomachs

Ants have one stomach where they hold and consume their own food and another stomach to hold food they share with other ants. This allows the ants that forage for food to feed the ants that remain in the nest to tend to the queen.

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Any cell can be poisoned. That includes any multicellular organism. The less surface you have per body-weight the better you generally do when poisoned. The smaller you are the less favorable your survival in exactly those scenarios. I cannot answer beyond that.