my understanding is that bonecarn allows the consumption of bones, and also implies a carnivore tag. because of default goblin ethics, sieges and ambushes are a very good source of food.
goblins, by default, have farming.... thing is, no seeds are embarkable, you have to harvest them. because of [BABYSNATCHER] no other races will trade without editing either those races, or removing babysnatcher, so any farming must be done by a player who has gathered plants. i think this may be because of the [USE_EVIL_PLANT] tag, and the lack of such plants.
by making vermin butcherable and giving kobolds the butcher skill, i think what is happening is that they are getting meat from the vermin. this is just speculation, as i have not tested this method, and probably will not till i have a dwarven fort built up enough to drive me to playing goblins again.
rats spawning in any non-freezing is indeed a problem, however, as goblins seem to home in on freezing biomes in which to build their towers. might just be a quirk of the RNG, but in all of the worlds i have genned, i see the highest concentration of goblins in freezing biomes. this my e because of the competition for more fertile land between the other races, and goblins having nearly no competition for freezing and evil biomes.