I have BRB and currently she is in a 25 qt tub on a non heated section on my rack. She is only a baby so the tub is probably a bit to big for her but she is doing really well in it. She is probably five-six months old, eating mice fuzzies and about two feet long. They do need super high humidity, as a baby 85-95% humidity, and as an adult 80-85%. I wouldn't put an adult in a rack tub, a biophile type cage would be the best cage for an adult. While they are on the skinner side for boas they do get pretty hefty, and depending on male or female they get over five-six feet easy. As an adult small mice would be to small, they would probably eat medium to large rats. You could feed one a small rat however you would have to feed a couple at a time.
I don't know how Brazilian was thrown into this I think the care of Colombian and Brazilian Rainbow Boas is pretty much the same, same size and everything. However I can only give experience of owning a BRB.