Cat question, please help?

KAY, my cat is 15-16 years old in human years. Is it true that if you give it shots (such as rabbi shots) it will affect it? because ive heard rumours at that old age it can kill them. ALSO is it bad to feed it different foods? because i heard its bad for cats to change their diet. She has been eating dry cat food all her life and I think its nice to give her tuna every now and then.