Yes, you can kill rabbits if you wish (and get some meat, which restores a bit of health); there are not that many in the game, anyway. I don't recall if rabbits turn hostile; they will hop away if there are 2 together and you kill one. IIRC rats are hostile to start with (though they do not attack you). Bread also restores a bit of health, and chicken restores mana (rats do not drop meat when defeated, and you probably wouldn't want to eat it if they did).
Primarily it is cats and dogs that people have an issue with turning hostile. Even if cats turn hostile the one in Aleroth that you can talk to will remain friendly. There is one hostile guard dog that can show up in a particular room (at an inn) if you do something in a certain way and then return.
IIRC I accidentally killed a cat trying to kill a rabbit before it did (it may not give much experience at the start of the game, but they were my experience points to take), but it deserved it so I didn't reload. I wouldn't have attacked dogs deliberately, but if they are going to attack me (starting at the inn) I didn't have a problem defending myself (and my first game I didn't realize the guard dog was always hostile, so I had the option to avoid returning if I wanted to forgo looting the place).
