The lionfish that I had (the one that ate my clown) was a FuManCu lionfish as well. He was easily 5" long. IME, they are bigger than 3" - I don't know where you got that 3" max length. He ate a tomato clownfish that was probably 2.5" in length.
But, I don't know, he seemed pretty tame most of the time. I had a scooter blenny in there too, and honestly, the blenny was the most incompetent swimmer, and just lazily swam in front of the lion all the time. He never got eaten.
But, IMO, if you care about the fish you have in there right now, I wouldn't add one. To me, it would just be too risky. Kinda like keeping a pet wolf or something. Never really quite tamed. If you are willing to risk the other fish (sounds like you are set on a lionfish of some sort) then go for it.