I don't feed medicated crumb as I don't want to rear birds which constantly need fixing. It's now pretty much well known that the types of bacteria animals (and humans) are exposed to very early in life is crucially important in developing a strong immune system and avoiding allergies. No antibiotic can be perfectly targeted and must kill much more important stuff in the bird's gut when they should be developing good gut flora.
OK, if you have an issue then solve it, but I don't believe in using antibiotics or other drugs as a protective measure, any more than as a growth hormone. Perhaps some folk have cocci on their land and I am just lucky as we have only ever lost one chick out of many hundreds reared (excluding hatching dramas) and he was suffocated we think by the other birds (probably my fault for having too many in the brooder). We have raised hundreds of chicks under broodies in the garden and never give them anything other than crumbs and then growers. We do prioritise fresh water and lots of clean bedding with DE and stuff.
We did also once buy a bag of growers from Cornwall Farmers (BOCM) I think and only noticed later that there is a meat withdrawal period after feeding it. To me, that's pretty awful.
We have given coxoid once to a Marans cockerel who was on deaths door and he's still with us today although I'm sure it affected his mind as he's often to be found upside down in a dust bath with both feet in the air, talking to himself.