Hi Emz, I drink Caffeine free diet coke, it has zero everything but does have citric acid. It has not affected me at all, I am in ketosis and have been for close on 3 weeks now, I have lost 32lbs in 22 days. I also believe that Coke Zero is OK as that does not have citric acid, citric acid supposedly is not good for Ketosis but it is trace amounts so I do not know what the issue is.
Anyway, try them out, you will find that it works or does not work. Your weight loss is not going to suffer for it at all, the diet coke and coke zero have no calories, no fat, no carbs. Checks the contents of each bottle/can and make your own mind up.