Rats are predators (of small things of course) in the wild and RELISH getting a live cricket or mealie. They can kill larger things, of course that's not something you'd want to allow as they don't need to eat something that big..but an adult rat (pet) has been known to kill things like pet birds (forget if a Cockatiel or Parakeet/Budgie?)..but they had a brief encounter and the rat quickly killed the bird, to the owner's dismay.
We used to mix Pretty Bird pellets, some human grade seed mixes, dry cereals,pastas,some dried veggies and beans..and dried fruits. That, and fresh fruits/veggies/little meat/cooked things from time to time...basically just a wide variety.