Not necessarily. It depends who initiated the contact - if Zimmerman was the initial aggressor (in the words of the Florida statutes, if he "initially provoke[d] the use of force against himself"), he cannot then use deadly force and claim self-defense, even if Martin had the upper hand in the fight.
Also, if Zimmerman was the initial aggressor, that effectively nullifies the "Stand Your Ground" law (e.g., if he was the initial aggressor, then even if he was in imminent danger of death, he could not use deadly force unless he had first "exhausted every reasonable means to escape such danger")
I think Zimmerman is toast. He was stalking his victim. Encountered him. Shot him. The rest of it is just window dressing.