It's possible with some considerations.
In June I added a small(2in) female G. lamarck to my 4-foot 120gal reef and she adapted very well.
As she has grown and established herself, she has become the dominant fish of the tank.
She harassed one of my leopard wrasses until it wouldn't come out of the sand bed and perished; and she still harasses the other periodically.
My first choice for a swallowtail angelfish was the Watanabei, but I settled for the Lamarck.
Both have the same misty blue body color with dark accents, but the price difference between the two species was very easy to consider going with the Lamarck.
And trying to get a small G. watanabei will take some time; as most available are already 3 inches or larger.
So if you do add a Genicanthus sp. Angelfish, make sure it is one of the last added.
Or any further additions are large enough to stand up to it.
Overall, I really like having the angelfish in the tank.
Hope this helps.