I'd go with gambusia vs. fhm if I was filling. Those fhm are going to be struggling, trying to breed and nest while water is continually rising. Males may stay and guard eggs if there are any until dead. Plus they will be reloacting looking for new cover continually. Gambusia are surface living live bearers. They could live in rising water forever. Still living on surface reproducing. I'd go with a baitfish that can grow and multiply while I was filling.
Not to be critical, but what is the point of having that deep of water? I have one pond made in a natuarally occuring valley with real steep sides. I graph it and almost nothing is below 10 feet.
