We make an effort not to use anything that states in its usage notes: 'Not Approved For Eggs That Will Be Used For Human Consumption' or 'Do Not Use On Food Producing Animals'. Since we are raising our flock holistically, I can't help you with dosage on terramycin. However, if you read the instructions on the terramycin package, it should give dosage rates.
I would try one thing, not all three. If you try all three at once, the vinegar will counteract the vitamins and electrolytes, you won't know what worked and you could do more harm than good.
If you're going to give the antibiotics, follow the recommended length of treatment (7 days or whatever it is). Then I would suggest you give some plain fat-free yogurt with active cultures after the terramycin treatment is over so you can rebuild the beneficial flora in the chick's digestive system.
If your chick is not eating, that is not a good sign. Try to mix some of his starter feed with warm vitamin/electrolyte water and get it into his mouth. You can try to massage his throat to help get food down if he doesn't seem to swallow on his own.
A vitamin/electrolyte solution is better than gatorade or Karo water. Gatorade is made with the sodium levels required by a human, not a chick. Salt poisoning could kill your chick, especially if it is weak already. Never mix vitamin/electrolytes with apple cider vinegar.
Apple cider vinegar doesn't need to be organic; It needs to have the mother. It will say on the bottle. If you want to raise your chicks organically, they need to be on certified organic chick starter immediately (I think it is by day 4 or 5) and you would not use drugs.
These are my personal comments, others will have their own views.
Remember, sometimes no matter how hard you try to keep them alive, some won't make it. Keep us updated and good luck.
regards,
keljonma