You need to know what commercial immunisation viruses they were given, if any. These are live viruses, given in sequence during rearing, which can flare-up with the stress of handling, transport and new surroundings. Sometimes they overwhelm the birds and they die, even healthy ones. Problem is if your existing flock isn't immunised with the same they will catch all these viruses simultaneously- this happened to us when we put hybrids into a pedigree flock. So you were right to keep them well away.
Antibiotics will not cure them, just control any secondary infections resulting. I think it is usual to give hybrids weak strains of IB, ILT and Mycoplasma now (airborne spray?), but I'm rather out of touch with that living here. The only immunisation here is Newcastle disease, compulsory for showing and which has to be renewed every year as it is a dead virus immunisation.