You don't actually need to mod vanilla files to get kobolds with dogs. Just give the dogs the "POISONOUS" creature class and leave off MAMMAL. The dogs will still be nonvenomous mammals, but the kobolds will think they are appropriate pets.
They won't be mounts, though.
What dogs should be given to the kobolds?
Hmm. You have a few options, you can either assign an existing breed to the kobolds or you can create a new fantasy breed of dog which was bred for their specific purposes.
I'm not sure which of the existing breeds is best for them, but if you were to go with the latter choice, I suggest a protective, long-furred, massive dog with poisonous saliva.
They would be rare and expensive, however, to counteract their size and many uses.
They would be a decent source of poison for the kobold's weapons, and their protective nature would give the fragile kobolds something more to fight with.
If you want you could go a bit farther and really make the removal of the mammal tag worth it, by giving them green scaly hides, flashing yellow eyes, and other traits better suited to a reptile.
Quite similar to a Komodo dragon in that respect.