Different 'antibiotics' act on different bacterium. There a basically gram-negative and gram-positive bacteria, and the different antibiotics usually act on one of the other.Your animals are so fortunate to have your
nursing skills I remember amoxicillin was used for one of my respiratory infections.
Are the 'cillins-related antibiotics for general body infections or are they for specific infections? What can tetracyclines be used for?
Amoxicillin is what is known as 'broad-spectrum' antibiotic. It will work with both types of bacteria - usually. It's in the same family as penicillin, usually used in respiratory infections, ear infections, etc.
Tetracycline are also a broad-spectrum antibiotic, it seems to be out of favour these days, likely due to resistance. It used to be used for treating coccidiosis in livestock, but now they are using Sulpha-Trim, that is what I used with the youngest chicks last year when they were ill.
I generally try not to use antibiotics, but with sever diarrhea, or a respiratory illness, retained eggs, lash egg, I will whack the antibiotics to them and not take any chances.
Last year Sophia became very ill and had respiratory issues, I assessed some sort of ascitis, when I drained her abdomen it was a terrible green/black fluid. I immediately put her on a heavy does of Amoxicillin for 10 days. She started feeling better after a couple days thankfully. I keep a very close eye on her, and she gets aspirin at night to help with any inflammatory response to any internal egg laying.