Check out littlebigcat.com. Dry food is NOT better for cats than wet food. It helps with tartar a tiny bit, but hardly any more than wet food. Kibble crumbles when it hits the tip of the sharp teeth -- so it does not really do that much scraping. And Cats are not designed to eat "kibble" by nature -- they are carnivores, designed to eat meat. It is better for them -- but all this is negated if you don't feed a high quality food. Some wet and dry foods that you feed your cat every day would be comparable to people eating McDonald's every day.
As for the mixture -- it's okay as long as it doesn't sit out. As mentioned, if he doesn't eat it all in a half hour, throw it away. When dry food gets wet, bacteria forms on the surface quickly.