Oh, that's interesting. We had a member with a bunny in GI distress, and she had avian/reptile Bene-Bac on hand for her birds. The only difference I could tell between avian and mammal is that one of the bacteria strains is specific to birds and reptiles. They have the same 4 bacteria, but the bird one has a bird-specific strain of one of the bacteria in it instead of the mammal-specific strain. I doubt the bird/reptile version would hurt a rabbit, but I figured they wouldn't get as much benefit from it because of that one different strain.
I don't know much about bottle-feeding babies, as I've never had to do it. I think any advantage that you can get to help them survive is worth trying, though, since it is so hard to bottle-feed them successfully.