Two thoughts on Gambusia:
1) If they are comfortable in 1/2 inch of water under a leaf, perhaps that's one reason they are so ubiquitous. Any time a pond has runoff in or out, Gambusia could be the first fish to swim in through temporary shallow water access. If Bullheads can live in wet grass, maybe Gambusia can swim through it.
2) Disregarding any particular source, Gambusia are more expensive than FH here up North.
ML has already been successful at convincing me to try Gambusia as an additional forage species at some point in the future when my pond is weedier and has more cover for them.