I have kept babies/subadults in CB70's with no real problems. Sure they have a tremendous amount of clutter and hiding material in each tub since...well they are far to big. But I always say it's better to go a little to big than to small especially for adults who can get decently big.
But correct me if I am wrong here a 20 gallon tank has far less floor space than a sweater box because it's sides are so high.
I certainly hope a CB70 isn't to small for adults that would really throw off my plan.