Not all drones 'hover', or are quad-copters. Most drone-swarms have been demonstrated using drones that fly like airplanes. 20 years ago when I started building RC planes, the smallest RC planes were about 6 inches wide. Many people are working on drones today that are insect-sized.
A mosquito-sized drone carrying a neurotoxin certainly would be used to carry out assassinations, crowd control, etc... And certainly would be a scary prospect.
Again, if the government is willing to use a mosquito drone full of neurotoxin to assassinate someone, why wouldn't they be willing to just send a man round with a pistol? The problem (in this example) is a government willing to assassinate political opponents, not that they have access to some fancy cyberpunk way of carrying out the hit.
A mosquito-sized drone carrying a neurotoxin certainly would be used to carry out assassinations, crowd control, etc... And certainly would be a scary prospect.