Hi C.J.,
I'm a big fan of YA and middle grade and, even though I'm kind of late, I wanted to chime in here. I agree with some of the other commenters, in that you may be limiting yourself by thinking about this too specifically, and that no book will be exactly like the one you are writing. However, I read your opening and I have some more suggestions for books you might want to look at if you haven't already.
Some really good middle grade books, specifically about sentient mice and rats, are
Mrs. Frisby and the Rats of Nimh and
Harry Cat's Pet Puppy. Both address issues of belonging, coexisting with humans, and living as outcasts while simultaneously striving for a solution to a larger problem.
Resentment, oppression, and forbidden friendship come into play in
Roll of Thunder, Hear My Cry (T.J. Avery's “friendship” with Melvin and R.W. Simms, vs. the Logan children's genuine friendship with Jeremy Simms), and in
Number the Stars, a book about friendship and loyalty in Nazi-occupied Denmark.
Annie on My Mind is written for an older readership, but it addresses some huge issues regarding social norms and ostracization (two high school girls fall in love and, in the face of social and family pressures, struggle to be faithful to their own feelings).
There is also a wonderful series of picture books by Graham Oakley, called The Church Mice Books. Most of them are out of print now, but a reasonably-sized library should still have them. Although they are intended for a younger audience, Oakley explores ideas about community and companionship, as well as providing a good dose of humor (and Best Pictures Ever

).
Anyway, I don't think you need to be looking specifically at middle grade Talking Animal books, but, as PeeDee pointed out, they're definitely around if you want them. I do think that you might be well served by looking at books addressing what happens when characters violate social constraints, and there are plenty of those too.
Good luck!