Depends on if they're fry or not...and how tiny of a shoehorn you can get...and how attached you are to the idea of them surviving for any length of time past a femtosecond. I would go with zero as I actually like fish, even if they have the misfortune of having been labeled as a hybrid.
Well...he doesn't say the flowerhorns are in the tank. And it seems actually physically impossible to have 3 in a 5G...unless they were fry...which they don't appear to be.
Also...at 40 bucks...makes me think is possible typo and he means 50/55. Certainly a possibility...before we all go flaming his email lol.