Honestly, a 40B is too small for tangs. Once they mature, they will outgrow the tank. I wouldn't add anymore unless they are very small and your ready to upgrade in the future or give them away.
Best way to introduce them is in an acclimation box. Then they can't hurt each other off the batt and can still see each other and adjust to having new inhabitants. You can also observe and see if aggression goes away and if not they are already caught and can be easily removed.