I'm not the expert in cast iron, but yes, a negative rake insert with no chipbreaker would be my choice. Cast iron is brittle, and will not benefit from a chipbreaker.
You may always suffer some breakout of the base metal where the cutter exits the work, so sometimes a special precautionary cut needs to be taken all around the part, so that the tool exits somewhere along the center of a side, rather than at an edge.
If you are milling cast iron near the surface of the casting, there may be some hardness, or inclusions that are hard on tool edges, too. Talk to your tool dealer about the proper grade of insert. There is so many changes in insert grades, I don't even attempt to keep up. I just ask for a higher grade number than was on the old box of inserts
