I have a large haddoni and a large gigantea in the same 40 breeder. So far I think all is well. When I first added the gigantea, less than 6 months age, it was pretty bleached. It has regained quite a bit of its zooxanthellae, but it has shrunk from about 16" down to about 12". It may have lost mass because of the lack of zooxanthellae, or maybe it has something to do with the haddoni??????? It's feeding response has improved ten fold, and the color has improved. I feed it every day, so hopefully it will start rebuilding mass soon. I've also kept a very small, about 3 or 4 inches, gigantea with my haddoni, and noticed no problems.
With chemical warfare and anemones things can be pretty tricky. There are many factors that can influence the outcome. Total water volume, flow direction, filtration, amount of water changes................. No one will be able to tell you that it will work out fine. I've kept this combination before without noticeable issues, but I'm still a little nervous about how it will work out in this system with these two monsters over the long term.