Is it okay to feed different brands of cat foods to my cats? And what about leaving out dry kibble for them during the day while I am gone?
There is absolutely nothing wrong with feeding a variety of brands and flavors of canned food to your cats. In fact, it is actually a good way to ensure they are receiving a well balanced diet. It is not a good idea, however, to free feed the dry kibble. In the long run you are not doing your cats any favors. Dry food is heavy on grains and cats are not designed by nature to digest grains. Feeding free choice also keeps them snacking all day which is not good for their digestive systems. Feeding free choice dry food can contribute to the development of bladder and kidney problems. Cats can go for many hours between meals and commonly do so in the wild.
Here are a couple articles that might be of interest:
Freeze dried foods are better than more highly processed dry kibble and canned foods, but are still not as beneficial as raw food. Here is one more link about feeding a raw diet to cats: http://catnutrition.org/