Respectfully, we stock for the tanks we have, not for the ones we may have later. Yes, the tank is 5 feet long, but (assuming you get them as juveniles) the Sailfin and Naso will require a larger tank in 2 years. As for the Blue, it will require a minimum of a 180 long-term, and I personally feel they should only be housed in an 8' tank or better.
You need to remember that it isn't just about the size of the specimen, but its behavior(s) as well. Failing to provide adequately for both is poor husbandry. We want the animals in our care to thrive, not just survive.