I hate to deflect, but this is something that your vet will need to address. The cat should be receiving sub-q or iv fluids, and what antibiotics are appropriate will depend on so many factors that only a vet should answer this for you. Under NO CIRCUMSTANCES would I recommend using any antibiotic on a cat with kidney issues and dehydration without a vet's prescription.
I WILL say that my kidney cats did receive antibiotics on occasion, but only ones prescribed by our Vet. AND we didn't let our cats get dehydrated. When they needed fluids, they received them. You can do sub-q fluids at home with a prescriptions for the fluids themselves, and can also give fluids via mouth with an eyedropper or needleless syringe. Adding extra water to their food is a good way to help with dehydration also