personally if you get a small one you can always exchange it down the road if it gets to big, i wouldn't do a purple tang without atleast a 55 gallon tank.
I have a 75 gallons tank (4feet by 18 x 20 inches I believe)
The macro algae appear to be 3 different types of caulerpa (Mexicana, Grape and varuiation of Mexicana)
Caulerpa algae produces caulerprin, which is a type of toxin. Some fish will nip on it, only to spit it out. Some will actually eat it, but IME, it is not usual. You may need to remove the algea by hand, without or with the help of any fish.
my sailfin and pajama tangs love bubble caulerpa. they won't touch chaeto, and certain types or hair algae- i didn't know there was more than one type of hair algae, but apparently, there is in my tank :?
you can put a tang in a 75, but he'll outgrow it eventually.