I know that this is an old thread but it's still not marked as answered.
Chroma key after initial color correction. you can always add an adjustment layer to be applied to the clip being keyed to fine tune it within the scene. It doesn't make a difference what you shoot in as long as its shot in color. makes no difference if the profile is flat or not... slog2..slog3..rec709.. ProRes.... doesn't matter... just color grade before you key..