neurotransmitter usage is prescribed at the stem cell level because all neurons within a single hemilineage express the same neurotransmitter. There does not seem to be coexpression of neurotransmitters (at least between glut, gaba, and ach), but some may coexpress monoamines.
gabaergic hemilineages
cholinergic hemilineages
glutamatergic hemilineages
references
Lacin, Haluk, Hui-Min Chen, Xi Long, Robert H Singer, Tzumin Lee, and James W Truman. “Neurotransmitter Identity Is Acquired in a Lineage-Restricted Manner in the Drosophila CNS.” eLife 8 (March 26, 2019): e43701. https://doi.org/10.7554/eLife.43701.