In trying to figure out where my head is at to day, here's the following.
Since a spell was cast that only targets Zada, Hedron Grinder, his triggered ability is placed on the stack on top of Soul's Fire.
In response to the triggered ability, Player sacrifices Goblin Flectomancer, changing the targets of Soul's Fire, such that Zada, Hedron Grinder will do Damage equal to its power to the opposing Player. Goblin Flectomancer resolves.
Triggered ability of Zada, Hedron Grinder tries to resolve. Will it check to see if the spell that was CAST only targeting Zada, Hedron Grinder still only targets Zada, Hedron Grinder?
If it doesn't, or is satisfied that while the spell currently targets Zada, Hedron Grinder and the opposing player it was CAST only targeting Zada, Hedron Grinder, it will then create a copy of Soul's Fire for each other creature Player controls.
Will the ability change only the first target, replacing Zada with another creature for each copy, or will it change both?