Getting older birds always carries the risk of bringing in pests and diseases.
Unless you've followed strict quarantine protocols,
there's a chance that your chicks have already been exposed to whatever is ailing the older birds.
Euthanizing the older sick birds, or taking them back to where...
I would not give probiotics and antibiotics at the same time,
think they may be counter-indicative, but not sure.
@Wyorp Rock might know.
@casportpony too...who wrote this excellent article about administering oral meds...
Glad to hear she made it.
I'd leave out all the herbs, garlic, ACV....just lots of clean cool water and good feed.
Not sure if you have Sav-A-Cik electrolytes/vitamins available there,
but it really helped mine thru the heat to give a dose of that every few days thru the heat waves. Saved one...