The red is probably iron and it isn't likely to hurt the fish. That is a large number of fish for a small pond. You will have to keep up with maintenance and keep an eye on oxygen levels. Of course, small is in the eye of the pond owner. In zone 6 you shouldn't have a bad problem with ice but you need to keep an air bubbler going.

I would doubt that it is iron. Iron as it oxidizes turn to what we call rust. Rust is an orange color and is not red. So I would say that it probably not iron. Since you said that the rock is eroding that would tend to lead me to believe that the rock you are talking about is sandstone. Sandstone could easily have red sediments in it or washing into a pool on the rock. Most other types of rocks are hard enough that water isn't going to leach or erode much out of them in a short period of time.
Mike