Why start them on antibiotics? Are they going to be hatched already sick? I don't use antibiotics until there is irrefutable evidence of the need, and even then which antibiotic works best for the particular problem has to be decided. Overusage of antibiotics when not needed simply leads to resistance to treatment when they actually are needed.
Are you perhaps referring to medicated feed? The only medication should be Amprollium, which builds resistance to cocci, an internal parasite. Even that use is optional, though I use it when I can find it, many don't.
Brooders should definitely be tested before hatch or chick arrival. Use a thermometer, or better still, several, to get the lamp height and heat distribution balanced before putting chicks in. It doesn't require 48 hours of preheating, but better to get it all balanced before putting the chicks in.
Congratulations on the Orps, they are quite nice.