Yes there is a difference in the dose. In dosage determination, several factors are considered i.e dose rate (how many mg per kg), concentration of drug and weight of animal. Considering broilers weigh more than chicks, the dose will differ in terms of the total volume. The BMR may not play a big role as the animals in question are of the same species but i think weight will play a bigger role.
in out door no difference and you need just to calculate the volume according to dose but in lab maybe it have different according to immunity system of the animal
I think it all depends on the route of administration of the antibiotics. With oral administration via drinking water what is considered is the quantity (weight) of the antibiotic and the volume of water in which the drug is to be suspended/dissolved. With parenteral administration (which is unlikely in chicks) the body weight is taken into consideration in calculating the dosage.
yes. Antibiotic residues are illegal in food animals and poultry have a very short life. Young chicks are fed antibiotics in feed but there is a cutoff point well before slaughter age is reached.