it would drown.
Actually, I think you'd be fine with a small rabbit, as long as you're prepared to give it up when it gets bigger. Tell you what, my rabbitfish is my favorite one, has a ton of personality, and I would have a hard time giving him up. He eats out of my hand, is very curious, and overall a very interesting fish. Mine is a "one-spot" - the yellow ones with the black spot on the back.
They are pretty active swimmers, so make sure you have a clear "run" in your tank for him to exercise. I'm planning on upgrading to a 120 next year, because they can get fairly large.