Of course smaller tangs can go in smaller tanks. As mentioned, is it really right to have it grow in that tank, and then just get rid of it later? Would you do this with a snake? a cat? a parrot? a dog? If you have another tank lined up in your own home, I see no problem with it, but many people "plan" on a larger tank and it never happens.
This issue is just most popular with tangs, but it happens with larger wrasse, triggers, rabbitfish, etc.
75 Gallon Reef