I'm not a fan of buying a small hippo for a few reasons:
1) Smaller hippos have a very low survival rate, especially with newer hobbyists, which is sounds like you are, please forgive me if I'm wrong.
2) You don't currently have another tank ready to go for when this fish reaches the size.
3) Hippos can get stressed easily, as many tangs can, and this can become much more of an issue in a smaller tank.
I don't really think there are any tangs that belong in a 50G breeder, especially not long term. I've seen a 11" one in captivity, although most reach about 10", some a little larger, some a little smaller.
You'd be much better suited with a dwarf angelfish.