As has been said, boas can and will eat rabbits. But there is no reason why they have to. I would only use a small rabbit for an exceptionally large female BCC. A BCI or Dumerils will do just fine on rats all their lives. If you really wanted a boa, if that was the only thing holding you back, get a boa - you won't be sorry. Plus you can look at some of the dwarf localities such as Hog Islands or Nicaraguans - many of them don't exceed 5-6 feet.

But if you would rather have a colubrid, take a look at black tailed cribos. They are very closely related to Indigos but you don't need a permit and they are much much more affordable. They are also highly intelligent and have strong personalities just like Indigos do.