The reason amoxicillin is considered dangerous for guinea pigs, hamsters and gerbils is because it kills intestinal bacteria, causing diarrhoea which may be severe. To be safe, I would ring the vet again and explain your concerns, and double check that they meant to prescribe amoxicillin. It may have been a mistake. Ask if they can prescribe a different antibiotic. This article (
Drug Therapy in Pet Rodents) lists some antibiotics that are considered safe for hamsters (amoxicillin is not among them).
Just out of curiosity, what were Luna's symptoms that led to her being diagnosed with renal failure? I've heard of it occurring it rodents, but I wasn't sure what symptoms it would cause? How was it diagnosed?