Maybe @Nambroth can explain this better than I can... Metronidazole will only work on certain types of bacteria, the others I mentioned will treat bacteria like e.coli, which is a very real possibility, especially if your hen has blackhead. Metronidazole is *not* a broad-spectrum antibiotic, the others are. FWIW, vets have told me to use Baytril with metronidazole, so I know that combo is okay. Cipro is in the same family as Baytril, so that's probably okay, too.
-Kathy
This is my understanding as well. According to my (experienced avian) vet, Baytril may be used effectively with Metronidazole/Flagyl. A full course of antibiotics is recommended when using Baytril, from my understanding, and generally should be given no less than 5 days consecutively (7 is ideal, 14 days for more severe problems).
As always, if advised otherwise by a vet, take their advice over mine, because I am NOT a vet.