How should I set my pets diet? Does it have anything to do with the breed?

It is my understanding that your pet's diet affects what stats are increased when your pet levels. This does not really vary across breeds. There are many different approaches to choosing a diet. More info on what foods affect which stat can be found here: http://beastkeeper.com/faq

If you'd like advice on how to choose a diet I would be happy to help. There may be some info in user guides, and I know there is info in past help threads. It really depends on your preference. Some people like to enhance certain traits, or create well-rounded pets. It is up to you :). If you don't really have a preference feel free to leave it balanced as is.

I hope that was able to help. If you still have more questions I would be happy to assist! :)