As far as I'm aware there are no direct flights between Porto Alegre and Iguazu. A couple of options are flying TAM connecting in Sao Paulo or Rio, or Gol connecting in Curitiba.

At Iguazu, note that there are 2 airports. You will arrive at IGU on the Brazilian side and continue your journey to Buenos Aires from IGR on the Argentine side so it would make sense to visit the Brazil side of the falls first when planning your trip.

Personally, I would stay a night in Iguazu to give yourself more time. I doubt you'll be able to arrive before midday with your flight connection and half a day really isn't enough to do this wonderful spectacle justice.