Haddon's have gotten a really bad rap. I have one that is about 12 - 15" in diameter and is hosting one of my Clarkii. I love them and am getting another one to host my other pair of Clarkii. My tank is fairly aggressive assortment including:
1 Humu Trigger (who actually tangled with the Haddon's over a piece of krill). He got stung but survived.
3 Pajama Cardinals
1 Lemon-peel Angel
A pair of Blackspot Angels (M & F)
1 Fork Tailed Blenny (2 1/2 " long)
1 Lawnmower Blenny
1 Snowflake Eel (also tangled very briefly and now stays far away)
and the 3 Clarkii
in my 120 Gal. Everyone respects it, even me. I've been stuck to it once when doing some maintenance.
They are actually quite beautiful.
Yes, they're aggressive, but so are most anemones. I had a BTA which killed my Mandarin, that swam into it. That's what they do. If your fish are small and wander into it, the sting can kill it. By the way, if newly metamorphosed clownfish wander into an anemone, even one that commonly hosts that species, they can become lunch. Clowns need time to acclimate to the host.
One of the reasons Haddon's are known as Saddle Anemone is because they are the natural host for Saddleback Clown's.