I have never heard of being too small for cocci meds? I would think to be on the small side they would want to start even sooner! How old is your baby? Usually it is suggested to only give shots to healthy puppies and it looks like they gave the Rabies shot way earlier than most would suggest and WAY too soon after the last puppy shot. This is way over doing his system. Do you leave food down for him all the time too? A pup of his size should have food available at all times. I would give something to put good bacteria in his belly too.
Good luck!!