It's easy, really. Feed them a food that they do well on, like to eat, and that you can afford. There is no one "perfect" food for any particular breed or mix. They will likely be on the smaller side of the medium range, since Poms are in the 6 to 10 pound range and Sibes are in the 40 to 50 pound range.
I see this particular cross, and always hope the Pom is the sire, and not the Sibe, because asking a toy sized bitch to whelp puppies sired by a much larger dog is so dangerous.
I only feed raw. It's very easy. My base mix is chicken thighs, chicken hearts, chicken bones, eggs, carrots and green beans. If your dog is a couch potato feed around 2% of body weight. Castrated dogs may eat even less. Intact and active dogs can eat up to 5% of their body weight per day. A good starting point is 2% of body weight, as this is what most pet dogs need to keep lean muscle.