That is a good point.
I think it might have been so. As several people have pointed out, the first Redwall book was a start of a long series, thus BJ was figuring everything out that he wanted to include or exclude. That's why we see mentions of the mice having farm animals, and other various flaws. Maybe Brian was planning on having a special ranking for the vermin, ie. maybe rats would be the lowest order and then ferrets, stoats, weasels, and maybe a wearet or a wolverine at the top, I don't know. Kind of like a food chain for animal rankings. Curious thought though. I think as he got more developed in the books, Brian decided to just have them all equal in rank instead of having different leveled classes.