Galaxia is pretty aggressive. If one stinger hits another coral it sends out more. I have yet to see it lose any fights. I think the current will also play a part in how far it extends. I guess I have not really given you an answer to a safe distance, I would keep an eye on it. HTH
My galaxea, before I got rid of that menace was about a foot away from its nearest neighbor. It STILL stung it. I believe galaxea should be in a very large tank with nothing closer than 12"-13". I gave it to someone with a barren 180 gallon tank.
I guess if you had the flow in the direction of where there arent any corals you might have better luck. So that the sweeping tentacles would blow away from any other corals. Those are some good pics Tim