One would need to be using Baytril INDISCRIMINATELY and for long term to cause Campilobactor resistance. Your one time use of it for an obvious illness which they were not willing to aggressively treat was justified. You chick is teeny tiny and cannot be allowed to just shrivel up and die.
No Baytril is not a permitted poultry antibiotic but you can bet my sweet little fluffy tush that is being used ‘off label’ all the time.
In fact 99% of the stuff we treat our poultry for is ‘off label’ and or illegal for use in poultry. La-te-da, if it were my chook I would treat. You did the right thing.
Furthermore I came across a study that was done for treating Mycoplasma and Baytril was more effective in treating it than the other drugs.
Many time Vets do not have the time to do individual research on treatments and thus do not know what is being done.
This article compared three different antibacterial agents against mycoplasma
https://pubmed.ncbi.nlm.nih.gov/18679898/m
Seems that the UK is further ahead of the US in poultry and Baytril
https://www.noahcompendium.co.uk/?id=-479259
And actually sulfa drugs are to treat coccidiosis not mycoplasma as far as I know of, Bob would know better likely.
And lastly here is some heavy ready regarding use of such said antimicrobial meds up for discussion here, nowhere do they ever mention campylobacter resistance….
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3801280/
ANYWAYS….
YOU DID THE RIGHT THING. Do not second guess yourself. The Vet is just covering her arse.