There were a couple of times that it did move...but that was right at the start.....well in the first couple of months or so. It would crawl on its side and move along till it got caught in a current and then do cartwheels in mid-water. So what I did was to put him in spot with a small rocky barricade around his base.....just 2-3 inches high. With the supporting wall it held itself upright and slowly worked its foot into the gravel....since then its not moved once and I have already moved the enclosing rocks. You could do a search with my name and there is an older LTA thread with some pictures that show the rocky barricade. Luckily for me when my LTA was caught in the current it could never get to an intake because the water current would just push him away. For a future setup I am going to perhaps modify so that I have only outlets in my tanks (no submerged powerhead). My pump will pump water right from the sump through a PVC pipe right into my tank. And so there will be no intakes that can be a danger. Just the overflow will be left, and I had never seen him float that high to be of any concern.
I dont think my anemones bother each other...atleast never happened physically....not sure if they do some chemical harm to each other though....