So all said and done, I'm leaning toward the tank not finished cycling and a rise in ammonia that did the fish in, and not the salinity of 1.026 you said you had
even after such an acclimation as the op described, going from 1.021 to 1.026 would be enough to stress (and possibly kill) the fish, especially if they were very small. when you are increasing salinity, it has to be done over a much longer period of time than if you are decreasing.
hang in there, op. if this is the worst mistake you make in this hobby, you'll be golden!