I don't know if my main drain is iron or not, but it's that old. All the plumbing I've been able to see at this point has been copper, not iron. Keep in mind that ships with tons of iron on them go to the bottom of the sea and stay in great shape for ages, and the salt concentration in the sea is 10x what will be in your pool. And, I've had iron pipes rust (sewer) in both my older homes, with no salt present. I'm thinking it's the combination of air and water that rust, not a constant underwater situation.
My opinion? I am not going to give you one. Contact the SWCG manufacturer and see what they say.
I put in my sig line that my pool is circa 1960. If it was built when the house was built, it was built in 1954. There was an addition put on my house, including the pool, not too much longer. So I don't know the exact age of my pool, but I can say between it and my irrigation system it's the most copper I've ever seen in my life!