If it was me, i would treat them all. I had 5 babies once and they all died because i treated them too late, about 3 weeks later my rooster showed signs so i took him out and treated him. He survived then another couple of weeks later a hen showed signed etc so i treated them all and had no more...
I have had some before that show the classic rattling and gurgling and no sneezing or coughing etc, believe me it will come in time. I made the mistake twice of not treating it.
My advice is get some Doxycyline as soon as ant treat them all at once. That way ones that have it yet are not showing...